请教一提,谢谢!

The police in Jamestown, Indiana have introduced a new crime prevention policy that includes random stopping of drivers, searching of houses, outlawing of guns, and other strict measures. While the constitutionality of this policy is debatable, the positive results of it are obvious, because crime has dropped 40% in one year.
Which of the following, if true, most weakens the conclusion of the argument above?
A) Prior to the establishment of the new crime prevention policy, the crime rate in Jamestown was already relatively low.
B) Within a year after the establishment of the new policy, the population of Jamestown dropped by 50%.
C) In another city, the same crime prevention tactics only reduced crime by 2%.
D) During the year prior to the establishment of the new policy, only two persons had committed murder in Jamestown.
E) Within a year after the establishment of the crime prevention policy, there was a 50% increase in police brutality cases.
对比下来就只能选B了，可是还不是很清楚，难道population dropped by 50%就一定对应crime也要dropped by 50%吗？
附：题目的解释 (B) If the population of Jamestown dropped by 50 percent, then a 40 percent drop in the number of crimes is not significant. Fewer residents in a town would naturally lead one to expect fewer crimes. The other answers do not directly refute the argument that the new policy has led to a reduction in crime.
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