Did you ever have someone's name on the tip of your tongue and yet you were unable to recall it? (21) this happens again, do not try to recall it. Do something (22) for a couple of minutes, and the name may come into your head. The name is there. Since you have met (23) person and learned his name. It only has to be dug out. The initial effort to recall (24) the mind for operation, but it is the subconscious (25) that go to work to dig up a dim memory. Forcing yourself to recall almost never helps because it doesn't loosen your memory; it only tightens it. Students find the preparatory method helpful (26) examinations. They read over the questions (27) trying to answer any of them. Then they answer first the ones (28) which they are most confident. Meanwhile, deeper mental activities in the subconscious mind are taking (29) ; work is being done on the more difficult question. By the time the easier questions are answered, answers to the more difficult ones will usually begin to (30) into consciousness. It is often just a question of waiting for recall to come to the memory.
21.
A. Whether
B. When
C. While
D. As