Studies have shown that levels of depression among those diagnosed with clinical depression actually increase in the springtime. The results of the studies were no different for subjects who spent most of their days outdoors than for those who stayed indoors. Researchers, once baffled by this finding, now hypothesize that seasonal changes in the mental health of the most severely depressed can be based on more than exposure to light.

Which of the following, if true, would most strengthen the researchers' hypothesis?
A. The same subjects evidence no change in mental health in the autumn when the amount of light is equivalent to that in spring.
B. As all people inherently believe that their health will improve in the spring, the unrealized expectation of improved health will often lead the most depressed to an even deeper level of depression.
C. Healthy subjects report feeling more alert and happy in the springtime as compared with the winter.
D. Seasonal-affective disorder (SAD) affects nearly one in ten people and can seriously interfere with an individual's work schedule and lifestyle habits.
E. Other scientists have demonstrated how over-exposure to sunshine can deplete the brain of the neurochemical seratonin and result in higher levels of depression.