A chemical spill in Oak City that took place in February tainted the

OFFICIAL EXPLANATION

The phrasing of this question looks tricky at first, but let's break it down. First we see a claim, made by several scientists, that a certain chemical spill in February may affect distant faraway water supplies. Then we see evidence from a study that same month that no pollution was found in these distant water supplies. Finally, we are asked to select a plausible reason that this evidence does not actually subvert or invalidate this claim. Why, in other words, does the evidence from the study not rule out the possibility of wide-scale contamination?
Only Choice (D) provides a realistic answer. If it takes a long time for the contaminants to spread, it may be too early, during the same month as the spill, to identify the full extent of the consequences. Choice (C) actually would undermine the claim, and so is incorrect; meanwhile, Choices (A), (B) and (E) are not relevant to the claim at all. Chemical spills may certainly be dangerous, difficult to clean, and they may contain chemicals used in paper production, but none of these facts affect whether water supplies near Oak City will be contaminated by the February spill.

The correct answer is Choice (D).