Man is the only animal that laughs. Why is this true？ What makes us respond as we【C1】______ to pleasurable experiences？ What is the history of this "happy convulsion" as someone once【C2】______ it. and just what is its function？
We are not short【C3】______ theories to explain the mystery, for centuries, biologists, psychologists, and medical men have【C4】______ a definitive explanation of laughter. One writer theoried that its function is to intimidate others or to gain stature【C5】______ them by humiliating them. Another took the opposite view that we laugh in order not to cry. A psychologist offered the explanation that laughter functions as a remedy【C6】______ painful experiences, and that it serves to defend a person【C7】______ what the psychologist termed "the many minor pains【C8】______ which man is exposed", that the seventeenth century, a writer set【C9】______ the theory that we laugh when we compare ourselves with others and' find ourselves superior; in effect, we laugh at the infirmities of others.
【C10】______ every theory has been concerned with either the structure or the function of laughter,【C11】______ relatively few have been devoted【C12】______ the question. of its origin. I propose to offer a theory which, so far as I am 【C13】______ , has not previously been set forth: that only those animals capable of speech are capable of laughter, and that therefore man, being the only animal that【C14】______ , is the only animal that laughs.
Those of us who have observed Chimpanzees closely feel quite confident that the chimpanzee occasionally【C15】______ behavior. that looks very much like a primitive precursor of human laughter. This behavior, however, has been observed only in a human【C16】______ ; whether or not it occurs under natural【C17】______ is dubious; but the【C18】______ fact that under any condition an ape is capable of such behavior. is【C19】______ more than passing【C20】______ for does it not indicate that early man had the rudiments of laughter？
【C1】

A. are
B. did
C. do
D. would