Please help

dakuandakuan

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?
AThe use of acoustic alarms increases the number of commercial fish caught by the fishing company's boats.
BWhen disoriented, harbor porpoises are not significantly more likely to be killed by other boats.
CEnvironmentalists advocate the use of acoustic alarms as a means of protecting the harbor porpoise population.
DThe alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters.
EThe cost of installing acoustic alarms on fishing boats is less than the cost of repairing nets damaged by harbor porpoises.
正确答案 B


为啥不是D   我觉得BD 比 因为上警告的时候如果海豚不够多，那么下降率是低了 可是海豚在那个海岸的数量也不多呀？ 所以觉得D比B 好， 但是不理解为什么是Ｂ　答案。