There are two factors which determine an individual's intelligence. The first is the sort of brain he is born【C1】______. Human brains differ considerably,【C2】______being more capable than others.【C3】______no matter how good a brain he has to begin with, an individual will have a low order of intelligence unless he has【C4】______to learn. So the second factor is what【C5】______to the individual--the sort of environment in which he is brought【C6】______. If an individual is handicapped(受阻得)【C7】______, it is likely that his brain will【C8】______to develop and he will never【C9】______the level of intelligence of which he is【C10】______ The importance of environment in determining an individual's intelligence can be【C11】______by the case history of the【C12】______twins, Peter arid John. When the twins were three months old, their parents died, and they were placed in【C13】______fostered(寄养的)homes. Peter was【C14】______by parents of low intelligence in an【C15】______community with poor educational opportunities. John,【C16】______, was educated in the home of well to do parents who had been to college. This environmental【C17】______continued until the twins were【C18】______their late teens, when they were given tests to【C19】______their intelligence. John's IQ(智商)was 125, twenty-five points higher than the【C20】______and fully forty points higher than his identical brother. 【C1】