CR -To prevent harbor porpoises

[Raja sekharS832]

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.

A, C, E are clearly out.
D- What if the alarms were installed at the time of year when harbor porpoises are NOT most plentiful in the Massachusetts waters. The harbor population was decreased well before the installation of alarms. So, the conclusion will fall apart. Please correct me . Official answer B.

[RonPurewal]

nope, D is irrelevant. the only thing D would affect is whether the alarms are saving proportionally fewer lives (if they're installed at non-peak population times), or proportionally more lives (if they're installed at peak population times).

that's not an issue here, since the conclusion is simply "The alarms ... are saving the lives of harbor porpoises in those waters". the truth/falsity of this statement is in no way affected by quantity.