The incidence of influenza cases increases during the winter. This illustrates... correlation only
causation only
both correlation and causation.
O neither correlation nor causation.

correlation only

Explanation:

This is the only correlation. It can't be causation because winter is not what causes influenza cases. It is what comes with winter that does. Staying indoors, different air humidity, temperature, etc. Not winter itself. And it is not both nor neither since it is the only correlation that can be seen in this example.

what effect does exercise have on the nervous system?

the answer is c.