In double-blind studies neither the test subject or the researcher knows who was given the drug and who was given the control. Why are these studies effective for testing a drug that may elevate mood and relieve feelings of depression?

Placebo effect

Explanation:

People tend to think they've been cured by something even if the something isn't effective, and they might even experience minor symptoms of having been effectively treated. If people know that the drug they're being given is the one being tested, they'll be more likely to experience this effect because it's the drug that the researchers think is going to work. Researchers conduct double-blind studies to prevent bias in the researchers and to prevent the placebo effect from interfering with the results from the test subjects.

D. The patients and researchers are not given this information so they cannot be influenced in reporting the effectiveness.