mallya12 wrote:
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?
The question asks "Which [answer choice] is an assumption on which the argument depends." So, we are looking for an answer choice that MUST be true in order for the author to reach his/her conclusion.
The conclusion is that "The alarms... are saving the lives of harbor porpoises in those waters."
Evidence for this conclusion is that "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."
With this in mind, take another look at (D):
Quote:
(D) The alarms were installed at the time of year when harbor porpoises are most plentiful in the Massachusetts waters.
Does this absolutely
have to be true in order for the author to reach his/her conclusion?
Think through the argument if the author does
not assume that the nets were installed during the time of year when porpoises are most plentiful. Perhaps the alarms were installed when the population was low, and then a bunch of porpoises arrived in the months after. This would
strengthen the evidence in the passage, because the number of porpoises caught in nets has decreased despite the overall population increasing. Because the argument does not fall apart if (D) is not assumed, (D) is not an argument on which the argument depends.
Note: this hypothetical situation is sufficient to eliminate (D), but you could think of many additional cases in which this answer choice impacts the conclusion in different ways.
Let's say that the alarms
were installed at the time of year when porpoises are most plentiful -- what does that do to the conclusion? It depends on several additional factors. Maybe a huge majority of the porpoises left the Massachusetts waters immediately after the alarms were installed, so the number getting caught in the company's nets would decrease with or without the influence of the alarms. This would
weaken the author's conclusion. Maybe the large number of porpoises did stick around -- if this were the case, the evidence in the passage would not be affected.
Overall, we cannot say how the information in (D) impacts the author's conclusion, so it is definitely not an assumption on which the argument depends.
I hope that helps!