单选题 .  In the same way that a child must be able to move his arms and legs before he can learn to walk, the child must physiologically be capable of producing and expei4encing particular emotions before these emotions can be modified through learning. Psychologists have found that there are two basic processes by which learning takes place. One kind of learning is called "classical conditioning" This occurs when one event or stimulus is consistently paired with or followed by, a reward or punishment. It is through classical conditioning that a child learns to associates his mother's face and voice with happiness and love, for he learns that this person provides food and comfort. Negative emotions are learned in a similar fashion.
    The second kind of learning is called "operant conditioning". This occurs when an individual learns to do things that produce rewards in his environment and learns not to do things that produce punishments. For example, if a mother always attends to her baby when he cries and cuddles him until he is quiet, she may teach him that if he cries he will get attention from mother. Thus, the baby will learn to increase his crying in order to have his mother more.
    Every day, we grow and have new experiences. We constantly learn by reading, watching television, interacting with other people, and so forth. This learning affects our emotions. Why is it that we learn to like some people and dislike others? If a person is nice to us, cares about us, we learn to associate this person with positive feelings, such as joy, happiness, and friendliness. On the other hand, if a person is mean to us, does not care about us, and even deliberately does things to harm us, we learn to associate this person with negative feelings, such as unhappiness, discomfort, and anger.1.  The author's main purpose in writing the passage is to ______.