This question is #28 in OG 2016. (Interestingly, this question is often cited as having the typo "weather" rather than "whether." It certainly hcan be read that way, but we should trust that GMAC has more diligent proofreaders than that. Instead, we should understand it as the effect that climate patterns have on crops).

This question is primarily testing IDIOMS.

The phrase "low enough that..." implies that a certain specific threshold has been reached: e.g. "you're not tall enough to ride this roller coaster," or "when the water level is low enough, you can walk across the sand bar."

In this sentence, there is no such specific threshold implied. Instead, the meaning suggests that reserves are generally low, and that has a causal relationship to prices. Eliminate any answer choices with "low enough": A and D.

The correct idiom is "so low that," not "so low such that." Eliminate B and E.

That leaves us with the correct answer: C.