The Fact That Bright Light May Be an Effective Treatment

The fact that bright light may be an effective treatment for seasonal affective disorder suggests that

A) this form of depression is produced by a malfunctioning biological clock that needs resetting.
B) seasonal affective disorder is a unique entity that should not be categorized with other forms of unipolar depression.
C) this is a not a real form of depression as any response to light is merely a placebo effect.
D) changes in circadian rhythms underlie most forms of depression.