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36． There is a great deal of geographical variation in the frequency of many surgical procedures—up to tenfold variation per hundred thousand between different areas in the numbers of hysterectomies， prostatectomies， and tonsillectomies．To support a conclusion that much of the variation is due to unnecessary surgical procedures， it would be most important to establish which of the following?
　　（A） A local board of review at each hospital examines the records of every operation to determine whether the surgical procedure was necessary．
　　（B） The variation is unrelated to factors （other than the surgical procedures themselves） that influence the incidence of diseases for which surgery might be considered．
　　（C） There are several categories of surgical procedure （other than hysterectomies， prostatectomies， and tonsillectomies） that are often performed unnecessarily．
　　（D） For certain surgical procedures， it is difficult to determine after the operation whether the procedures were necessary or whether alternative treatment would have succeeded．
　　（E） With respect to how often they are performed unnecessarily， hysterectomies， prostatectomies， and tonsillectomies are representative of surgical procedures in general．
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答案是 B 为什么

39. The level of lead contamination in United States rivers declined between 1975 and 1985. Federal regulations 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 rives 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 contamination occurred between 1980 and 1985.

(C) Levels of lead contamination in rivers fell sharply in 1975-1976 and rose very slightly over the next nine years.

(D) Levels 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.

答案是B 为什么

感谢大牛求解！