Explanation

Reading the question: the prompt is a bit hefty, so we go to the question stem. This is an "explain" question. So there will be two things in seeming contradiction. We look for the transition from thing 1) to thing 2): the word "nevertheless." Thing 2) is shorter, so we start there. The mystery is that companies did not deplete their reserves even when prices were high. Why is that so crazy? Thing 1) will tell us: it's because companies keep their reserves for this purpose, so they can overall buy at lower prices. We will paraphrase the mystery as: "didn't use when high." The correct answer will resolve the mystery and be compatible with all of the facts in the prompt.

Applying the filter: (A) sounds reasonable to us at first as an explanation. But the prompt leads us to understand that companies would still want to deplete their reserves even if prices will be high in the future. If they thought prices would be higher next year, that would be a good reason to wait, but (A) doesn't say that. (A) is out. (B) doesn't explain "didn't use when high"; even if they need less oil than usual, they would still save by not having to buy when price are high. (B) is out. (C) doesn't explain "didn't use when high"; it gives a further reason for them to use now, so as not to buy now and save the buying for when prices are better. (C) is out. (D) is a good explanation for "didn't use when high"; companies generally did use when high, on average, but not all companies had to. It's a surprise solution but consistent with the data in the prompt. (D) is in. (E) is quite similar to (D) and quite close, but it's not as good an explanation.

Logical proof: we can analyze by cases to see that (D) is superior to (E). Namely, (E) could still be true even if prices were high for all companies involved; in such a case, (E) would limit the mystery, but it wouldn't explain the mystery. The correct answer is (D).