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　　The stratospheric ozone layer is not a completely uniform stratum, nor does it occur at the same altitude around the globe. It lies closest to the Earth over the poles and rises to maximum altitude over the equator. In the stratosphere, ozone is continuously being made and destroyed by natural processes. During the day the Sun breaks down some of the oxygen molecules to single oxygen atoms, and these reacting with the oxygen molecules that have not been dissociated, form ozone. However, the sunlight also breaks down ozone by converting some of it back to normal oxygen. In addition naturally occurring nitrogen oxides enter into the cycle and speed the breakdown reactions. The amount of ozone present at any one time is the balance between the processes that create it and those that destroy it.
　　Since the splitting of the oxygen molecules depends directly upon the intensity of solar radiation, the greatest rate of ozone production occurs over the tropics. However ozone is also destroyed most rapidly there, and wind circulation patterns carry the ozone-enriched upper layers of the atmosphere away from the equator. It turns out that the largest total ozone amounts are found at high latitudes. On a typical day the amount of ozone over Minnesota, for example, is 30 percent greater than the amount over Texas, 900 miles farther south. The density and altitude of the ozone layer also change with the seasons, the weather, and the amount of solar activity. Nevertheless, at any one place above the Earth’s surface, the long-term averages maintained by natural processes are believed to be reasonably constant.
　　The amount of ozone near the Earth is only a small percent of the amount in the stratosphere, and exchange of molecules between the ozone layer and the air at ground level is thought to be relatively small. Furthermore, the ozone molecule is so unstable that only a tiny fraction of ground-level ozone could survive the long trip to the stratosphere, so the ozone layer will not be replenished to any significant degree by the increasing concentrations of ozone that have been detected in recent years near the earth’s surface. The long-term averages of ozone both near ground level (ground level: n.[地质]地水准平面) and in the stratosphere are regulated by continuous processes that are constantly destroying and creating it in each of these places. This is why scientists are so concerned about human beings injection into the stratosphere of chemicals like nitrogen oxides, which are catalysts that facilitate the breakdown of ozone. If the ozone layer is depleted significantly, more ultraviolet radiation would penetrate to the Earth’s surface and damage many living organisms.
　　17. The passage suggests that factors contributing to the variation in the amount of ozone above different areas of the Earth’s surface include which of the following?
　　I. Some of the ozone found at higher latitudes was produced elsewhere.
　　II. There is usually a smaller amount of naturally occurring nitrogen oxide over high latitudes.
　　III. The rate of ozone production over the poles is less than that over the tropics.
　　(A) II only (B) III only (C) I and II only (D) I and III only (E) I, II, and III

　　18. Which of the following best states the central idea of the passage?
　　(A) Naturally occurring nitrogen oxides, as well as those introduced by humans, threaten to deplete the layer of ozone in the stratosphere. (B) A delicate but reasonably constant balance exists between the natural processes that produce and those that destroy ozone in the stratosphere. (C) There is little hope that the increased concentrations of ground-level ozone observed in recent years can offset any future depletion of stratospheric ozone. (D) Meteorologically induced changes in the concentration of ozone in the stratosphere tend to cancel themselves out over a period of time. (E) Solar radiation not only produces and destroys zone but also poses a hazard to human life.
　　19. The processes that determine the amount of ozone in a given portion of the stratosphere most resemble which of the following?
　　(A) Automobile emissions and seasonal fog that create a layer of smog over a city (B) Planting and harvesting activities that produce a crop whose size is always about the same (C) Withdrawals and deposits made in a bank account whose average balance remains about the same (D) Assets and liabilities that determine the net worth of a corporation (E) High grades and low grades made by a student whose average remains about the same from term to term
　　20. According to the passage, which of the following has the LEAST effect on the amount of ozone at a given location in the upper atmosphere?
　　(A) Latitude (B) Weather (C) Season (D) Ground-level ozone (E) Solar activity
　　21. The author provides information that answers which of the following questions? I. What is the average thickness of the stratospheric ozone layer? II. Why does increased exposure to ultraviolet radiation damage many living organisms? III. What is the role of oxygen in the production of stratospheric zone?
　　(A) I only (B) II only (C) III only (D) I and II (E) II and III
　　22. In explaining what determines the amount of ozone in the stratosphere, the author describes natural processes that form
　　(A) an interactive relationship (B) a reductive system (C) a linear progression (D) a set of randomly occurring phenomena (E) a set of sporadically recurring events