求解：prep07-29题

素儿

To prevent harbor porpoises from getting tangled in its nets and suffocating, a fishing company installed acoustic alarms on all its boats that fish in waters off Massachusetts.  The sound emitted temporarily disorients the porpoises and frightens them away.  Since the installation of the alarms, the average number of porpoises caught in the company's nets has dropped from eight to one per month.  The alarms, therefore, are saving the lives of harbor porpoises in those waters.

Which of the following is an assumption on which the argument depends?

(A) The use of acoustic alarms increases the number of commercial fish caught by the fishing company's boats.
(B) When disoriented, harbor porpoises are not significantly more likely to be killed by other boats.
(C) Environmentalists advocate the use of acoustic alarms as a means of protecting the harbor porpoise population.
(D) The alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters.
(E) The cost of installing acoustic alarms on fishing boats is less than the cost of repairing nets damaged by harbor porpoises.

我明白答案（B）的意思，但是之前选（D)因为是想要排除他因，不是因为海豚本身数量少导致缠在网上的少，从而证明报警器起作用了......要从save lives的角度来说，扯到安装时间上我是有点儿自high了，不过D也不是完全米道理的吧？最近总是混乱，求大牛支招~~~

sdcar2010

LZ, you need to understand the meaning of "necessary assumption."

D is not necessary for the conclusion of the passage to hold.

francislaw

Also, the conclusion is to "saving the lives of harbor porpoises in those waters". D sounds very WEAK in this respect, whereas B is much more direct and stronger.

素儿

谢谢楼上两位啦~~B说的是结论的必要条件