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passage 4
By 1970 it was well established that ultraviolet light from the sun contributes to skin cancer. Fortunately, much of the sun's most damaging ultraviolet radiation is screened out by a thin, diffuse layer of ozone—a toxic form of oxygen—in the stratosphere, 10 to 25 miles above the earth's surface.
During the 1970s, however, public policy makers worldwide were alerted to the fragility of the ozone layer through the pioneering research and advocacy of two Nobel Prize-winning scientists, Mario Molina and F. Sherwood Rowland. In the absence of pollutants, stratospheric ozone concentrations should remain stable over time, with natural production and destruction of the gas in rough equilibrium. Molina and Rowland showed how manufactured chlorofluorocarbons (CFCs)—highly volatile chemicals, millions of tons of which had been used each year in products such as aerosol sprays and refrigerants—chemically attack and deplete the ozone layer, diminishing its effectiveness as a shield against ultraviolet radiation. Studying two freon gases—types of CFCs—they observed that, when released into the lower atmosphere (troposphere), these gases slowly diffuse upward into the stratosphere. There, subjected to massive ultraviolet radiation, they break down into their constituent elements, including chlorine. The resulting increase in the concentration of chlorine in the stratosphere is devastating to the ozone layer. Chlorine and ozone chemically react in a way that both destroys the ozone and regenerates the chlorine atoms. As a result of this chemical reaction, each chlorine atom could destroy as many as 100,000 ozone molecules before becoming inactive.
In 1974 the two scientists estimated that the atmosphere contained the accumulation of five years of global CFC production. This meant that, given the rate of diffusion and breakdown of CFCs in the atmosphere, the depletion of the ozone layer would continue for years, if not decades, even if the production and use of CFCs were to cease immediately. Recognizing this as a pressing environmental threat, Molina and Rowland became public advocates for a prompt and proportionate public policy response. As a result, Molina was invited to testify before the U.S. Congress and was later appointed to the U.S. National Science Foundation Committee on Fluorocarbon Technology Assessment.
Predictably, the work of Molina and Rowland and their advocacy of dramatic policy changes were subjected to attacks by critics, especially scientists with ties to the CFC industry. However, over time their views were corroborated, especially by the discovery of a hole in the ozone layer over Antarctica, and this led to the development of an international agreement (the Montreal Protocol of 1987) to ban the production of ozone-depleting gases. In North America, CFCs were banned in the late 1970s, leading to a transformation in packaging for consumer spray products and the development of more environmentally friendly refrigerant chemicals.
27.
Which one of the following statements is most strongly supported by the information in the passage?
a
No refrigerant chemicals other than CFCs had been discovered when Molina and Rowland suggested that CFC production cease.
b
Refrigerant chemicals developed as substitutes for CFCs after 1987 release fewer chlorine atoms into the stratosphere than CFCs do.
c
CFCs were originally used in refrigeration components because they provided the most energy-efficient means of refrigeration.
d
The Montreal Protocol led to the cessation of CFC production in North America.
e
Some of the refrigerant chemicals being manufactured today contain chemicals known to be environmentally damaging.