单选题 The ozone layer, the fragile layer of gas surrounding our planet between 7 and 30 miles above the earth"s surface, is being rapidly depleted. Seasonally occurring holes have appeared in it over the Poles and, recently, over densely populated temperate regions of the northern hemisphere. The threat is serious because the ozone layer protects the earth from the sun"s ultraviolet radiation, which is harmful to all living organisms. Even though the layer is many miles thick, the atmosphere in it is tenuous and the total amount of ozone, compared with other atmospheric gases, is small. Ozone is highly reactive to chlorine, hydrogen , and nitrogen. Of course chlorine is the most dangerous since it is very stable and long-lived. When chlorine compounds reach the stratosphere, they bond with and destroy ozone molecules, with consequent repercussions for life on Earth. In 1958, researchers began noticing seasonal variations in the ozone layer above the South Pole. Between June and October the ozone content steadily fell, followed by a sudden increase in November. These fluctuations appeared to result from the natural effects of wind and temperature. But while the low October levels remained constant until 1979, the total ozone content over the Pole was steadily diminishing. In 1985, public opinion was finally aroused by reports of a"hole"in the layer. The culprits responsible for the hole were identified as compounds known as chlorofluorocarbons, or CFCs. CFCs are compounds of chlorine and fluorine. Nonflammable, nontoxic and noncorrosive, they have been widely used in industry since the 1950s, mostly as refrigerants and propellants and in making plastic foam and insulation. In 1989 CFCs represented a sizable market value at over $1.5 billion and a labor force of 1.6 million. But with CFCs implicated in ozone depletion, the question arose as to whether we were wiling to risk an increase in cases of skin cancer, eye ailments, even a lowering of the human immune defense system—all effects of further loss of the ozone layer. And not only humans would suffer. So would plant life. Phytoplankton, the first link in the ocean food chain and vital to the survival of most marine species, would not be able to survive near the ocean surface, which is where these organisms grow. In 1990, 70 countries agreed to stop producing CFCs by the year 2000. In late 1991 , however, scientists noticed a depletion of the ozone layer over the Arctic. In 1992, it was announced that the layer was depleting faster then expected and that it was also declining over the northern hemisphere. Scientists believe that natural events are making the problem worse. The Pinatubo volcano in the Philippines, which erupted in June 1991, released 12 million tons of damaging volcanic gases into the atmosphere. Even if the whole world agreed today to stop all production and use of CFCs, this would not solve the problem. A single chlorine molecule can destroy 10, 000-100, 000 molecules of ozone. Furthermore, CFCs have a lifespan of 75 - 400 years and they take ten years to reach the ozone layer. In other words, what we are experiencing today results from CFCs emitted ten years ago. Researchers are working hard to find substitute products. Some are too dangerous because they are highly flammable; others may prove to be toxic and to contribute to the greenhouse effect—to the process of global warming. Nevertheless, even if there is no denying that atmosphere is in a state of disturbance, nobody can say that the situation will not improve, either in the short or the long term, especially if we ourselves lend a hand.

单选题 As it is described in the passage, the major function of the ozone layer is closest to that of______.

单选题 The word "tenuous"(Para. 2)most nearly means______.

【正确答案】 D

【答案解析】解析：由题干定位到原文第二段第一句“Even though the layer is many miles thick，the atmosphere in it istenuous and the total amount of ozone，compared with other atmospheric gases，is small．”可知虽然臭氧层比较厚，但是里面的空气和臭氧的数量相对于其他大气层而言是很稀少的。文中tenuous意为“稀薄的；纤细的”，A项hazy意为“朦胧的，模糊的”；B项tense意为“紧张的，紧绷的”；C项clear意为“清楚的”；都不合文意。D项thin意为“薄的，细的”，与tenuous意义相当。另外，由句中的“even though”也可以推测所选答案应与“thick”意义相反，故答案为D。

单选题 Which of the following does the passage imply about the"seasonal variations in the ozone layer"(Para. 3)observed by scientists in 1958?

【正确答案】 C

【答案解析】解析：由题干定位到原文第三段“In 1958，researchers began noticing seasonal variations in the ozone layerabove the South Pole．Between June and October the ozone content steadily fell．followed by a sudden increasein November．These fluctuations appeared to result from the natural effects of wind and temperature．But whilethe low October levels remained constant until 1979，the total ozone content over the Pole was steadilydiminishing．In 1985，public opinion was finally aroused by reports of a‘hole’in the layer．”可知，1958年科学家们在南极开始观测臭氧层的季节性变化，他们发现在6月到10月期间臭氧会持续下降然后到11月开始陡然增加。这段时问全球气候、风向、温度等都会发生相应的变化。然而从1979年以后，整个臭氧层都在持续变少，到1985年，已经形成了我们所知的臭氧层黑洞。故答案为C。