Physicians prescribe drug A for a large group of depressed patients. Six weeks later they switch to drug B for every patient who did not respond to drug A. Many of these patients show improvement over the next few weeks. What conclusion, if any, follows?

a. Drug B is more effective than drug A.

b. Some people respond to drug B but not to drug A.

c. Any switch in drugs increases patients’ motivation and therefore helps them recover.

d. None of these conclusions follows.