(C)
The way in which memory takes place is not known. Some psychologists (心理学家) think that there is a physical change in the brain ceils or nerves when learning takes place.
Psychologists are also unable to say what makes people forget. Some things are forgotten quickly while others are always remembered. Sometimes a person seems to have forgotten certain material completely, yet is able to relearn the material more quickly than when he first learned it. In trying to understand memory, psychologists have done a lot of experiments.
Psychologists have found that the conditions under which something is learned have an effect on how well it is remembered. When a great deal of material is to be learned, for example, to learn one thing at a time is better than to learn many things at a time. A person then learns more quickly and remembers better.
Learning two similar things one after the other seems to have a bad effect on memory. When a person memorizes two sets of historical dates, one after the other, for example, he may mix up the two--the learning of new material prs the remembering of the already learned material.
Most forgetting takes place in the first day or so after learning. After that, forgetting happens more slowly. Material that is understood--such as ideas or riddles (谜语)--is not so easily forgotten. Very little forgetting takes place in time if an idea is well understood. The more meaningful the learned material is, the better it will be remembered. (C)The passage is mainly concerned with ______.