Official Solution:
Each spring, Northbridge University runs a weeklong alumni phone-a-thon. In a typical year, the phone-a-thon results in about 400 alumni signing up to mentor current students. Therefore, if the university were to discontinue the phone-a-thon, the number of alumni who sign up to mentor students each year would decline by about 400.
Which of the following is an assumption on which the argument depends?
A. The phone-a-thon costs the university less than the value of 400 alumni mentors’ time.
B. The university would still be able to contact alumni through email if the phone-a-thon were discontinued.
C. Few alumni who currently sign up to mentor through the phone-a-thon would sign up through other university outreach efforts if the phone-a-thon were discontinued.
D. The alumni who sign up to mentor through the phone-a-thon are not substantially more likely than other alumni to sign up to mentor in any given year, regardless of how they are contacted.
E. The university’s other mentoring sign-up channels would not become significantly more effective at attracting mentors if the phone-a-thon were discontinued.
The argument concludes that if Northbridge University discontinued its alumni phone-a-thon, the yearly number of alumni who sign up to mentor students would decline by about 400. The evidence given is that, in a typical year, the phone-a-thon results in about 400 alumni signing up to mentor.
To move from “the phone-a-thon produces 400 sign-ups” to “removing it will reduce total sign-ups by 400,” the argument must assume that these 400 sign-ups would not still occur through other routes. If many of the same alumni would sign up via email campaigns, social media, or departmental outreach once the phone-a-thon ended, then the evidence about the phone-a-thon’s contribution could remain true, but the total number of mentors would not drop by about 400.
(A) Incorrect. Whether the phone-a-thon is cost-effective has no bearing on the predicted size of the decline. The conclusion concerns how many mentors there will be, not whether the program is worth its cost.
(B) Incorrect. The existence of other ways to contact alumni does not have to be assumed for the conclusion to follow. Even if email contact were impossible, the key question would still be whether the 400 phone-a-thon mentors would be replaced elsewhere.
(C) Correct. If few of the alumni who currently sign up through the phone-a-thon would sign up through other outreach efforts if the phone-a-thon disappeared, then eliminating the phone-a-thon would remove roughly those 400 sign-ups from the yearly total. The argument therefore depends on this “no substitution” assumption.
(D) Incorrect. This claims that phone-a-thon sign-ups are not more likely than other alumni to mentor regardless of contact method. But the argument doesn’t need that. Even if phone-a-thon recruits are especially predisposed to mentor, the conclusion could still be true so long as they would not sign up through other channels once the phone-a-thon is gone.
(E) Incorrect. This is a classic GMAT trap answer choice (brings a weaken argument to the assumption fight) and is one step removed from the argument. We are focusing on the mentors and this answer choice talks about channels that recruit these mentors, implying that perhaps something may happen, but as you will see, this "assumption" requires other assumptions to actually hold true, which disqualifies it. This option says other channels would not become significantly more effective if the phone-a-thon ended. But that is not sufficient by itself and therefore not required as some other channels could become more effective while yet other channels would become less effective and still the total might still fall by 400 if the 400 phone-a-thon would be mentors do not switch to those channels. Moreover, we do not know the total number of mentors - what if the total is 401? A more potent assumption would basically say that "Other cannels can become more effective enough to recruit additional 400 mentors". The argument above only needs to rule out substitution by those mentors, not improvements in outreach in general.
Answer: C