This stimulus is a classical issue of forgetting about the middle ground!
The core of the argument can be seen as this:
Some teachers claim (love the classic LSAT "some people claim"....we can anticipate a differing view from this set up!) that students would not learn the curriculum without grade incentives.
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Students with intense interest in the material would learn it without the incentive.
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Students lacking all interest in the material would not be affected by the incentive.
------------> THEREFORE...
Incentive grades serves no necessary academic purpose.
This conclusion is a strong one. No essential academic purpose.
The evidence for this?
Kids that have intense interest and kids that lack all interest do not need this incentive.
What about the middle ground between intense interest and lack of all interest? Say, semi-interested? Does one have to fall into that dichotomy laid out before us? No!
(A) is wrong because it is out of scope (B) is wrong because the prompt doesn't take this for granted at all. The prompt only says that students who lack all interest in the material will be indifferent to their grades - (B) simply changes this and says students who are indifferent to their grades are interested in their material. (C) out of scope - prompt only talks about grades in terms of an academic purpose (D) it would be nice to think this, but out of scope (E) correct! As timmydoeslsat, the argument misses out the middle ground.