阅读下列短文，从每题所给的四个选项（A、B、C、D）中，选出最佳选项，并在答题卡上将该项涂黑。
A
There are two factors which determine an individual's intelligence. The first is the sort of brain he is born with. Human brains differ greatly, some being more capable than others. But no matter how good a brain he has to begin with, an individual will have a low level of intelligence 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 disabled environmentally, it is likely that his brain will fail to develop and he will never gain the level of intelligence of which he is capable.
The importance of environment in determining an individual's intelligence can be proved by
