September 2009, Sec1, #12. Please help me understand the right answer/diagram this

LSAT Determined

Hello,

I'm having trouble with understanding how to properly diagram this to get to the right answer. I initially chose A, but apparently the correct answer is D. But I'm not sure as to why.

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This is a sufficient assumption question.

Some people oppose the city's purchase of this thing. Those people oppose the purchase because art critics are debating over whether the thing is actually art. But, the purpose of art is to cause debate. Thus, it qualifies as art.

What I am looking for: The whole first half of the passage is context. The real argument starts with sentence 2's "But...". Essentially, our premise is that the purpose of art is to cause debate (diagrammed out: Purpose of art--->Debate. We have debate. Therefore, art. We need either debate--->art or purpose of art--->art.

Answer A: Not cause debate--->Not art. This is the inverse (incorrect negation) of what we want.

Answer B: The certainty of experts isn't relevant/not sufficient.

Answer C: This is pretty tricky since it is what you might conclude if you don't read carefully. The argument doesn't argue what should or should not be done. We only care about defining art. Thus, this isn't a sufficient assumption.

Answer Purpose of art--->Art. This is perfect, and it is one of the things I anticipated.

Answer E: This is just like C. We don't care about what should/should not be done.

Hope this helps.