The correct answer is (C).
(C) Assumption (Sufficient)
Step 1: Identify the Question Type
The "conclusion . . . follows logically if . . . assumed" formulation indicates a Sufficient Assumption question.
Step 2: Untangle the Stimulus
The stimulus presents two groups of aluminum cans: group M and group L. The conclusion states that group M contains twice as many cans as group L. The evidence for this is rather convoluted, so break it down piece by piece.
First, standard aluminum cans contain the same amount of aluminum. Both groups M and L contain standard aluminum cans (therefore, the amount of aluminum in each can in each group is the same). Second, half of the aluminum used to make the cans in group M came from the recycled cans of group L. Third, all the cans in group L were recycled and turned into cans in group M. Finally, materials other than aluminum in cans are insignificant.
Step 3: Make a Prediction
If half of the aluminum in group M came from the entirety of group L, then half came from elsewhere. In other words, group M must contain twice as much aluminum as group L did. The author thinks this means group M contains twice as many cans at group L did. Indeed, the logic seems sound. So, there must be something very basic to this calculation that is being assumed because the numbers seem to add up correctly. The author must be making a subtle scope shift between double the aluminum and double the cans. Look for an answer choice that, if added, proves that these two are actually equivalent. The correct answer may do this by eliminating an overlooked reason why they would not be comparable.
Step 4: Evaluate the Answer Choices
(C) is correct. Sure, the numbers add up correctly, but only if it is as straightforward as it seems. The ignored possibility is that some aluminum is lost during the recycling process. In that case, the number of cans in group M would not be exactly twice as many as in group L. However, once this choice confirms that that possibility is not occurring, it proves group M has twice as many cans. The hint at this assumption in the stimulus was the evidence that a negligible amount of other materials are added. That could have led you to ponder whether any material was lost.
(A) is Out of Scope. Whether the group M cans could be recycled further does not at all affect the comparison between the number of cans in each group.
(B) also is Out of Scope. The aluminum's quality is irrelevant to the numbers comparison. The evidence says cans do not vary in the amount of aluminum they contain, regardless of what grade it is.
(D) is Out of Scope in the same way that (A) is. The stimulus is restricted to group L's transformation into group M. The future of group M-in answer choice (A)-and the past of group L-in this answer choice-are both irrelevant.
(E) is an Irrelevant Comparison to cans of other materials. The ease of recycling does not affect whether the math adds up nicely.
