GMAT Critical Reasoning (CR) | Re: Verbal Focus Q: Layers of the earth’s crust are exposed

let us take a look at the answer options -

A - this cannot be inferred from the paragraph provided.
We have no idea about the actual numbers of mollusks.

B - Incorrect.
this directly contradicts the assertion made in the paragraph -
"can only survive at least 500 feet below the surface of the ocean."

C - Incorrect.
We do not whether they were the first, second or third people to see the exposed fold of the crust.

D - Correct answer.
the exposed fold of crust had to be at least 500
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