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.

B is correct because it addresses the issue of alarm saving lives.

D might serve as an explanation for why the company has caught less of porpoises, but does not address the saving their lives part.

I doubt if this is a text from GMAT.
I feel comfortable to assume that such a question is a creation of GMAT Tutors/ Private Organizations that prepare these questions for students benefit.

If this is infact a GMAT question, i expect it to be more precise as in option B.
There is an ambiguity here, other boats?? -- what are other boats?? -- 1)other boats of the same company or 2)other boats of different company that did not install the alarms??

Depending on the above, i would go with D if 1, i.e the other boats are mentioned as bpats of the same company, bcoz in such a case, B looses its importance as the average is already mentioned down to 1 from 8, so D is better as it supports the facts with its info.

But if other boats mean boats of other companies, I would go with B........ again if it says that other boats have no alarms installed on it.

This is my opinion, and if there are any experts around here, please thrash me if you feel my reasoning is wrong.

The conclusion here is that the alarms are saving lives.
B - If the alarm is causing harbor porpoises to get killed by other boats, then ultimately the alarms are not saving lives.

D - If we negate this option, we cannot say for sure whether the alarms are saving lives or the population is less in general and therefore not many porpoises are getting killed.
does not have the impact that B has.

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.

Type - Assumption
-Fishing company has installed alarms on boats
-Sound emitted temporarily disorients the porpoises and frightens them
- Monthly Average number of porpoises caught in nets has dropped from 8 to 1 .
Conclusion - Alarms are saving the lives of porpoises

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. Irrelevant

B. When disoriented, harbor porpoises are not significantly more likely to be killed by other boats. Correct answer - if this is true , the alarms are not saving the lives of porpoises

C. Environmentalists advocate the use of acoustic alarms as a means of protecting the harbor porpoise population. Irrelevant - the opinion of Environmentalists does not matter

D. The alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters. Incorrect - the quantity does not matter as long as the alarms are saving the lives the porpoises .Also , even if we negate this , the alarms were installed at the time of year when porpoise population was below the peak , the alarms might still help in saving lives .

E. The cost of installing acoustic alarms on fishing boats is less than the cost of repairing nets damaged by harbor porpoises. Irrelevant

Answer B
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Negation of B directly destroys the cocnlusion. But how to eliminate option D?

The conclusion is based on the premise that propoises caught has dropped from 8 to 1. What if the alarms were insatalled when the propoises were not plentiful and the number drop is due to less propoises.

Please explain why D is wrong?