填空题

There are two factors which determine an individual’s intelligence. The first is the sort of brain he is born with. Human brains (36) considerably, some being more capable than others. But no matter how good a brain he has to begin with, an individual will have a low order of intelligence unless be has (37) to learn. So the second factor is what (38) to the individual--the sort of environment in which he is reared. If an individual is hindered environmentally, it is (39) that his brain will fail to develop and he will never (40) the level of intelligence of which he is capable.
The (41) of environment in determining an individual’s intelligence can be (42) by the case of the history of the identical twins, Peter and Mark. Being identical, the twins had identical brains at birth, and their growth (43) were the same. When the twins were three months old, their parents died, and they were placed in separate [foster](收养的)homes. (44) .Mark was reared in the home of wealthy parents
who had been to college. (45) .This environmental difference continued until the twins were teenagers, when they were given tests to measure their intelligence. Mark’s I.Q. was 125, (46) .