大全－11－15

The level of lead contamination in United States rivers declined between 1975 and 1985. Federal legulation requiring a drop in industrial discharges of lead went into effect in 1975, but the major cause of the decline was a 75 percent drop in the use of leaded gasoline between 1975 and 1985.

Which of the following, if true, best supports the claim that the major cause of the decline in the level of lead contamination in United States river was the decline in the use of leaded gasoline?

A The level of lead contamination in United States rivers fell sharply in both 1975 and 1983

B Most of the decline in industrial discharges of lead occurred before 1976, but the largest decline in the level of river contamnation occured between 1980 and 1985

C Level of lead contamination in river fell sharply in 1975-1976 and rose very slightly over the next nine years.

D Level of lead contamination rose in those rivers where there was reduced river flow due to drought

E Although the use of leaded gasoline declined 75 percent between 1975 and 1985,80 percent of the decline took place in 1985.

ans:B

why? Thanks.

本人也百思不得其解，看上去B算是比较有logic的。

A.把单独两年抽出来，没人性

C.还说升高，必错没戏

D.说到drough，与gasoline 无关

E. 是在否定gasoline的作用

无奈下选B,请高手指点!!!!!!!!!!!

(个人感言：其实这种题满无聊的，ACT不会出的）

两种污染：工业排放和含铅汽油

B把工业排放减少的时间，前置，而把主要的污染水平下降，后置。

这样可以在时间上，把二者的因果关系分开，从而起到排除他因、加强另一因果的作用，GMAT非常喜欢用时间分离的方法排除因果。

75年法律生效-->工业含铅污水排放减少-->铅污染减少

80--85年不知什么原因--〉铅污染减少的更多

75-85含铅汽油减少使用75%也就是说80--85这段时间是在汽油使用的时间段里，有题可知导致铅污染的只有污水和汽油两个原因，因此推测出导致80--85这段时间铅污染减少的原因只能是减少含铅汽油使用这个原因，更可以进一步推出，80-85这段时间减少含铅汽油使用的程度超过75-79这段时间含铅汽油使用的程度。

这样可以解释的通