题目内容
（请给出正确答案）
Tiny drops of weak sulfuric and nitric acids are formed when .
A. sulfur oxides and nitrogen oxides react with each other
B. fossil fuels are burned in the open air
C. sulfur oxides and nitrogen oxides react with the water vapor and oxygen in the atmosphere
D. sulfur dioxide and nitrogen dioxide combine with each other in lakes or ponds
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If pollution continues to increase at the present rate, formation of aerosols (悬浮微粒) in the atmosphere will cause the onset (开始) of an ice age in about fifty years' time. This conclusion, reached by Dr. S. Rasool and Dr. H. Schneider of the United States Goddard Space Flight Centre, answers the apparently conflicting questions of whether an increase in the carbon dioxide content of the atmosphere will cause the Earth to warm up or increasing the aerosol content will cause it to cool down. The Americans have shown conclusively that the aerosol question is dominant.
Two specters haunting conservationists have been the prospect that environmental pollution might lead to the planet's becoming unbearably hot or cold. One of these ghosts has now been laid, because it seems that even an increase in the amount of carbon dioxide in the atmosphere to eight times its present value will produce an increase in temperature of only 2℃. which would take place over several thousand years. But the other problem now looms larger than ever.
Aerosols are collections of small liquid or solid particles dispersed in air or some other medium. The particles are all so tiny that each is composed of only a few hundred atoms. Because of this they can float in the air for a very long time. Perhaps the most commonly experienced aerosol is industrial smog of the kind that plagued London in the 1950s and is an even greater problem in Los Angeles today. These collections of aerosols reflect the Sun's heat and thereby cause the Earth to cool.
Dr. Rasool and Dr. Schneider have calculated the exact effect of a dust aerosol layer just above the Earth's surface in the temperature of the planet. As the layer builds up, the present delicate balance between the amount of heat absorbed from the Sun and the amount radiated from the Earth is disturbed. The aerosol layer not only reflects much of the Sun's light but also transmits the infrared radiation from below. So, while the heat input to the surface drops, the loss of heat remains high until the planet cools to a new balanced state.
Within fifty years, if no steps are taken to stop the spread of aerosols in the atmosphere, a cooling of the Earth by as much as 3.5℃ seems inevitable. If that lasts for only a few years it would start another ice age, and because the growing ice caps at each pole would themselves reflect much of the Sun's radiation it would probably continue to develop even if the aerosol layer were destroyed.
The only bright spot in this gloomy forecast lies in the hope expressed by Dr. Rasool and Dr. Schneider that nuclear power may replace fossil fuels in time to prevent the aerosol content of the atmosphere from becoming critical.
The author's main purpose in writing the article is to warn of ______.
A．warm weather
B．hot weather
C．a new ice age
D．a new iceberg
It is widely believed that the pull of gravity on a falling raindrop changes （） round shape into a teardrop shape.
A.of the drop
B.the drop' s
C.drop of
D.drops the
Bluetooth
When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a community of electronic devices. These devices communicate with each other using a variety of wires, cables, radio signals and infrared light(红外线) beams, and an even greater variety of connectors and plugs.
The art of connecting things is becoming more and more complex every day. And Bluetooth has now become a new popular method of connecting devices that can streamline the process. A Bluetooth connection is wireless and automatic, and it takes small-area networking to the next level by removing the need for user intervention and keeping transmission power extremely low to save battery power.
Bluetooth Operation
Bluetooth networking transmits data via 10w-power radio waves. It communicates on a frequency of 2.45 gigahertz(千兆赫)(actually between 2.402 GHz and 2.480 GHz, to be exact). This frequency band has been set aside by international agreement for the use of industrial, scientific and medical devices(ISM).
A number of devices that you may already use fake advantage of this same radio-frequency band. Baby monitors(微型监视器), garage-door openers and the newest generation of cordless phones all make use of frequencies in the ISM band.
One of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of about 1 milliwatt. By comparison, the most powerful cell phones can transmit a signal of 3 watts. The low power limits the range of a Bluetooth device to about 10 meters(32 feet), cutting the chances of interference between your computer system and your portable telephone or television. Even with the low power, Bluetooth doesn't require line of sight between communicating devices. The Walls in your house won't stop a Bluetooth signal, making the standard useful for controlling several devices in different rooms.
Bluetooth can connect up to eight devices simultaneously. With all of those devices in the same 10-meter(32-foot) radius; you might think they'd interfere with one another, but it's unlikely. Bluetooth uses a technique called spread-spectrum.(扩展频谱) frequency that makes it rare for more than one device to be transmitting on the same frequency at the same time. In this technique, a device will use. 79 individual, randomly chosen frequencies within a designated range, changing from one-to another on a regular basis. In the case of Bluetooth. the transmitters change frequencies 1,600 times every second, meaning that more devices can make full use of a limited slice of the radio spectrum. Since every Bluetooth transmitter uses spread-spectrum transmitting automatically, it's unlikely that two transmitters will be on the same frequency at the same time. This same technique minimizes the risk that portable phones or baby monitors will disrupt Bluetooth devices, since any interference on a particular frequency will last only a tiny fraction of a second.
Piconets(个人域网络)
Let's say you have a typical modern living room with the typical modern Stuff inside. There's an entertainment system with a stereo, a DVD player, a satellite TV receiver and a television; there's also a cordless telephone and a-personal computer. Each of these systems uses Bluetooth, and each forms its own piconet to talk between the main unit and peripheral(外围设备).
The cordless telephone has one Bluetooth transmitter in the base and another in the handset. The manufacturer has programmed each unit with an address that falls into a range of addresses it has established for a particular type of device. When the base is first turned on, it sends radio signals asking for a response from any units with an address in a particular range. Since the handset has an address in the range, it responds, and a tiny network is formed. Now, even if one of these device
A．Y
B．N
C．NG
A、the tests become more difficult
B、the test fee has increased
C、the total number of college students drops
D、it’s difficult for law school graduates to fi nd jobs
Bluetooth
When you use computers, entertainment systems or telephones, the various pieces-and parts of the systems make up a community of electronic devices. These devices communicate with each other using a variety of wires, cables, radio signals and infrared light(红外线) beams, and an even greater variety of connectors and plugs.
The art of connecting things is becoming more and more complex every day. And Bluetooth has now become a new popular method of connecting devices that can streamline the process. A Bluetooth connection is wireless and automatic, and it takes small-area networking to the next level by removing the need for user intervention and keeping transmission power extremely low to save battery power.
Bluetooth Operation
Bluetooth networking transmits data via low-power radio waves. It communicates on a frequency of 2.45 gigahertz(千兆赫)(actually between 2.402 GHz and 2.480 GHz, to be exact). This frequency band has been set aside by international agreement for the use of industrial, scientific and medical devices(ISM).
A number of devices that you may already use take advantage of this same radio-frequency band. Baby monitors(微型监视器), garage-door openers and the newest generation of cordless phones all make use of frequencies in the ISM band.
One of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of about 1 milliwatt. By comparison, the most powerful cell phones can transmit a signal of 3 watts. The low power limits the range of a Bluetooth device to about 10 meters (32 feet), cutting the chances of interference between your computer system and your portable telephone or television. Even with the low power, Bluetooth doesn't require line of sight between communicating devices. The walls in your house won't stop a Bluetooth signal, making the standard useful for controlling several devices in different rooms.
Bluetooth can connect up to eight devices simultaneously. With all of those devices in the same 10-meter (32-foot) radius, you might think they'd interfere with one another, but it's un- likely. Bluetooth uses a technique called spread-spectrum(扩展频谱) frequency that makes it rare for more than one device to be transmitting on the same frequency at the same time. In this technique, a device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis. In the case of Bluetooth, the transmitters change frequencies 1,600 times every second, meaning that more devices can make full use of a limited slice of the radio spectrum. Since every Bluetooth transmitter uses spread-spectrum transmitting automatically, it's unlikely that two transmitters will be on the same frequency at the same time. This same technique minimizes the risk that portable phones or baby monitors will disrupt Bluetooth devices, since any interference on a particular frequency will last only a tiny fraction of a second.
Piconets(个人域网络)
Let's say you have a typical modern living room with the typical modern stuff inside. There's an entertainment system with a stereo, a DVD player, a satellite TV receiver and a television; there's also a cordless telephone and a personal computer. Each of these systems uses Bluetooth, and each forms its own piconet to talk between the main unit and peripheral(外围设备).
The cordless telephone has one Bluetooth transmitter in the base and another in the handset. The manufacturer has programmed each unit with an address that falls into a range of ad- dresses it has established for a particular type of device. When the base is first turned on, it sends radio signals asking for a response from any units with an address in a particular range. Since the handset has an address in the range, it responds, and a tiny network is formed. Now, even if one of these devices should receive a signal
A．Y
B．N
C．NG
In 2008, scientists encountered the top of the ionosphere all______.
A．thanks to the weak solar activity
B．because of the advanced CINDI
C．by chance
D．as predicted
A．by environmentalists concerned about air pollution
B．only by people who sought after fashion
C．in rural areas poorly served by public transportation
D．in big cities crowded with street cars
A．women can provide better care for children
B．women are too weak to do any agricultural work
C．women are biologically suited to domestic jobs
D．women cannot compete with men in any field
Immune-regulated diseases are related to stress because such diseases______.
A．tend to produce a stress-related chemical
B．will affect a person's character severely
C．will make people weak in body and soul
D．tend to undermine the response to stress
