Since the passage of the state’s Clean Air Act ten years ago, the level of industrial pollutants in the air has fallen by an average of 18 percent. This suggests that the restrictions on industry embodied in the act have worked effectively. However, during the same period the state has also suffered through a period of economic decline. The number of businesses in the state has fallen by 10 percent, and the number of workers employed has fallen by 12 percent. It is probable that the business decline, rather than the regulations in the act, is responsible for at least half of the decline in the pollution. 13. Which of the following, if true, would most seriously weaken the conclusion drawn in the passage above? (A) During the last ten years, economic conditions in the nation as a whole have been worse than those within the state. (B) Amendments to the Clean Air Act that were enacted six years ago have substantially strengthened its restrictions on industrial air pollution. (C) Of the businesses that ceased operating in the state during the last ten years, only 5 percent were engaged in air-polluting industries. (D) Several large corporations left the state during the last ten years partly in order to avoid compliance with the Clean Air Act.（C） (E) Due to its small budget, the state office charged with enforcement of the Clean Air Act has prosecuted only two violators of the law since its passage. 这题答案为C，但觉得恨不严密，C的前提是这5%的公司产生的污染不起重要的作用，可是文中又没提到，所以当初犹豫了下还是选了D，D隐含的意思是这些离开的大公司释放的污染不符合规定，等于是说规定造成了污染的减少，我觉得这里D比C好，请指教！ PS 大全的A~D套题做的那个叫郁闷啊,错的找不着北了 | 
