Scientists have wondered for a long time if animals can learn to talk as human beings do.

Several years【C1】______ , two psychologists took a baby chimpanzee into their home and raised it as a member of their family. After six years, the chimpanzee had learned to say only four【C2】______ . Apparently, chimpanzees can not talk because they care physically unable to shape their mouths and move their tongues【C3】______ people do. 【C4】______, other scientists have been very successful in teaching chimpanzees to communicate in the sign language that is used by【C5】______ people. One chimp, a female named Washoe, has learned more than 160 signs. She knows signs【C6】______ nouns, verbs, adjectives, and various other parts of speech. She understands the meaning of the signs and【C7】______ to apply them to new situations.【C8】______ , she knows that the sign for "tree" means not just one tree, but all trees. Several other chimpanzees are now being taught sign language. They are beginning to use signs to communicate with each other【C9】______ with their human trainers. Scientists are【C10】______ to see what will happen to an entire colony of chimpanzees if they learn sign language and use it among themselves.

【C1】

A．after

B．later

C．ago

D．before