求助贴！prep2008 cr2 77题！

77.
Citizens of Parktown are worried by the increased frequency of serious crimes committed by local teenagers. In response, the city government has instituted a series of measures designed to keep teenagers at home in the late evening. Even if the measures succeed in keeping teenagers at home, however, they are unlikely to affect the problem that concerns citizens, since most crimes committed by local teenagers take place between 3 p.m. and 6 p.m.
Which of the following, if true, most substantially weakens the argument?
A. Similar measures adopted in other places have failed to reduce the number of teenagers on the streets in the late evening.
B. The crimes committed by teenagers in the afternoon are mostly small thefts and inconsequential vandalism.
C. Teenagers are much less likely to commit serious crimes when they are at home than when they are not at home.
D. Any decrease in the need for police patrols in the late evening would not mean that there could be more intensive patrolling in the afternoon.
E. The schools in Parktown have introduced a number of after-school programs that will be available to teenagers until 6 p.m. on weekday afternoons.

答案为什么是B啊？我觉得D很对的。。。