PT56, S2, Q8 This boulder is volcanic in origin and yet the

[ManhattanPrepLSAT1]

8. (D)
Question Type: Weaken the Argument
It is critical to note in this argument that the glaciers that covered this area were southward-moving. We are asked to undermine the conclusion that this specific boulder was deposited here by a southward-moving glacier. To conclude that it was deposited by a southward-moving glacier, we would need to assume that there is a geological source of volcanic rock somewhere to the north. If we attack this assumption, we can undermine the argument completely. Answer choice (D) accomplishes this.

(A) is a tempting answer, but not the correct answer. The truth of this answer makes it unlikely that the rock came from hundreds of miles away, but not impossible. The modifier "most" still permits for some boulders to have been moved more than 100 miles. Compare this with answer (D), which destroys the argument completely.
(B) references the closest source of volcanic rock (50 miles south). This rock couldn’t have been moved to its present location from that source, but it could have been moved from another source that lies hundreds of miles to the north. The existence of a close source to the south doesn’t impact the argument in any way.
(C) references a potential source 50 miles north. It’s possible that the rock in question is from this source, but the existence of this source doesn’t really weaken the possibility that the rock came from a source even further north (hundreds of miles north).
(E) doesn’t weaken the argument. Just because there are no other rocks of volcanic origin within 50 miles of this particular rock doesn’t mean that this rock wasn’t moved there by a glacier from a volcanic source that exists hundreds of miles to the north.