Answer: E

The explanation for the decrease in class size is "a decrease in mobility among the elderly during the winter months." We're looking for a choice that suggests that this decrease in mobility really occurs, and that it is limited to the winter months as the argument claims.

Choice (A) strengthens the argument, but not the explanation: If there are fewer classes and class size is still smaller, there must be a dramatic drop in the number of people who attend such classes. (B) is irrelevant, as we're concerned with month-to-month differences. (C) is irrelevant as well, since we don't know where these classes are offered.