The correct answer is (D).
Getting to the Answer: The paragraph poses many questions, but the correct answer will be the main idea of the paragraph that connects all of them: what makes a desert? Expect that the correct answer will summarize this point.
(A) Incorrect: while one of the questions in the paragraph does specify Death Valley, this does not summarize the main question posed by the paragraph
(B) Incorrect: while the paragraph talks about geophysical factors, this does not summarize the main question posed by the paragraph
(C) Incorrect: the paragraph does not ask how long it takes for a desert to form
(D) Correct: this summarizes the main question posed by the paragraph
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