The amount of sleep a human being needs depends on【C1】______ , the individual and possibly

The amount of sleep a human being needs depends on【C1】______ , the individual and possibly race. For example, doctors think that pre-school children need between 10 and 12 hours a night; school children between 9 and 11 hours; and adults between 7 and 9 hours. There are exceptional【C2】______ of old people who only sleep between 2 and 3 hours a day and continue to be active and healthy. The sleep requirements of different【C3】______ also appear to be different. Japanese people, for example, sleep fewer hours than Europeans. It is not known for【C4】______ if mental activity(apart from dreaming)occurs when a person is asleep.【C5】______ , it is certainly true that some people can wake up at a specific, predetermined time. There are also【C6】______ about mathematicians who solve difficult problems during sleep, because their【C7】______ minds continue working on the problem. Some types of unpleasant dreams(or "nightmares")are quite【C8】______ ; the dreamer is taking a test, but is not properly prepared; he is falling from a tree;【C9】______ an animal or thing is chasing him. Some people say these last two occur【C10】______ mans ancestors lived in trees and were in constant danger from wild animals.

【C1】

A．gender

B．occupation

C．age

D．personality

请帮忙给出正确答案和分析，谢谢！