@cynthia-lee since this is a strengthen question, we are looking for a flaw/weakness in the argument that an answer choice helps address. The flaw here is a classic correlation vs. causation issue. All we know from the study is that snoring is correlated with throat abnormalities. But since correlation does not prove causation, we do not know that snoring causes those abnormalities.
Therefore we should be looking for an answer choice that addresses that flaw. An answer choice that either 1) shows snoring causes throat abnormalities or 2) eliminates other explanations. In this case, (E) knocks out a very plausible alternate explanation, namely that throat abnormalities actually cause snoring (i.e. the causation runs in the reverse).
(C) is a prerequisite for running a valid study, but it does nothing to address the correlation vs causation flaw. In other words, the logical gap between correlation and causation still exists whether or not we check the boxes off on standard scientific research protocol
