II only
III only
I and II only
I and III only
I, II, and III
18. Which of the following best states the central idea of the passage?
Naturally occurring nitrogen oxides, as well as those introduced by humans, threaten to deplete the layer of ozone in the stratosphere.
A delicate but reasonably constant balance exists between the natural processes that produce and those that destroy ozone in the stratosphere.
There is little hope that the increased concentrations of ground-level ozone observed in recent years can offset any future depletion of stratospheric ozone.
Meteorologically induced changes in the concentration of ozone in the stratosphere tend to cancel themselves out over a period of time.
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?
Automobile emissions and seasonal fog that create a layer of smog over a city
Planting and harvesting activities that produce a crop whose size is always about the same
Withdrawals and deposits made in a bank account whose average balance remains about the same
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