A Friend States He Is Thinks He Is Mildly Depressed

A friend states he is thinks he is mildly depressed but he's not sure, and he asks if there is any kind of laboratory test that could determine whether or not someone is depressed.What would you tell him?

A) Cortisol levels are decreased in depression; he could have his cortisol levels checked by a blood test.
B) Currently, there is no way of diagnosing depression with a laboratory test.
C) The dexamethasone suppression test (DST) can be used only to diagnose severe cases of depression.
D) The dexamethasone suppression test (DST) is a biological test for depression.