[单项选择]Afterwards there was just a feeling of {{U}}let-down{{/U}}.
A. excitement
B. anger
C. calm
D. disappointment

Depression isn’t just feeling down. It’s a real illness with real causes. Depression can be set off by stressful life events, like divorce or a death in the family or it can appear suddenly, for no apparent reason. Some people think you can try to get yourself out of a depression. That’s not true. Many doctors believe that one thing that may cause depression is an imbalance of serotonin (血清素) —chemical in your body. If this happens, you may have trouble sleeping. Feel unusually sad or irritable. Find it hard to concentrate. Lose your appetite. Lack energy or have trouble feeling pleasure. These are some of the symptoms (症状) that can point to depression—especially if they last for more than a couple of weeks.
To help fight depression, the medicine doctors prescribe now most often is Prozac. Prozac isn’t a "happy pill". It won’t turn you into a different person.
Some people do experience mild side effects,