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根据下面内容，回答176-180题
There are two factors which determine an individual's intelligence．The first is the sort ofbrain he is born with．Human brains differ considerably, some being more capable than others．Butno matter how good a brain he has to begin with, an individual will have a low order of intelli-gence unless he has opportunities to learn．So the second factor is what happens to the individual--the sort of environment in which he is brought up．If an individual is handicapped environmentally,it is likely that his brain will fail to develop and he will never attain the level of intelligence ofwhich he is capable.
The importance of environment in determining an individual's intelligence can be demonstrat-ed by the case history of the identical twins, Peter and Mark．Being identical, the twins had iden-tical brains at birth, and their growth processes were the same．When the twins were three monthsold, their parents died, and they were placed in separate foster
A. human brains differ considerably
B. the brain a person is born with is important in determining his intelligence
C. environment is crucial in determining a person's intelligence
D. persons having identical brains will have roughly the same intelligence