The first thing you have to do in order to solve any CR question is to CLEARLY understand the prompt. From understanding the prompt all else follows.
In this case the point of the prompt is that solar energy becomes economically viable, meaning that using it costs the same as or less than using oil, at $35 per barrel of oil, and that in spite of the fact that solar energy has become more cost efficient, that price has not changed.
Now to the question.
Which of the following, if true, does most to help explain why the increased cost-efficiency of solar power has not decreased the oil price at which using solar becomes economically viable?
I guess we have to find something that has also changed, keep that threshold price at $35 even though solar energy is more cost efficient.
Likely what has changed is something about oil is used, but I am not going to waste time thinking about what might have changed, as there are five answer choices right there, and one of them is correct.
(A) This answer is tempting. "Ahah. The cost of oil is lower. THAT'S it." But wait a minute, the point is not that oil is currently cheaper to use than solar power. The point is that the economic viability threshold price hasn't changed. The current price of oil has no effect on the economic viability threshold price.
(B) This one is obviously wrong. If the price of solar power equipment has gone down, it's down, and the cost of raw materials is irrelevant.
(C) This has to be it. This answer choice indicates that using oil has become more cost efficient too. So as solar became more efficient, oil also become more efficient, and the economic viability threshold price remained the same.
Still, going to go through the last two choices, JUST to be sure.
(D) Ok, another obviously wrong answer. We are talking about solar versus oil. The fact that there are more of other types of plants is irrelevant.
(E) Haha. Another trap answer. Sounds interesting, but once again, this is about the price where solar becomes competitive with oil, and the factors affecting the supply of oil and the level of the price of oil don't affect the level of that economic viability threshold price.
So, yes, the correct answer is C.
