阅读下面材料，在空白处填入适当的内容(1个单词)或括号内单词的正确形式。

        There are two factors ____(determine) an individual's intelligence. The first is thesort of brain he is born ____. Human brains differ ____(consider), some beingmore capable than others.

____howevergood a brain he has to begin with, an individual will have a low intelligenceunless he has opportunities to learn. So the second factor is what happens tothe individual—the sort of environment ____ he is brought up. If an individualis handicapped(受阻碍)environmentally, itis ____(like) that his brain will fail to develop and he will never attain thelevel of intelligence of which he is capable.

        Theimportance of environment in determining an individual's intelligence can____(show) by the case of the twins, Peter and Mark. When the twins were threemonths old, ____parents died, andthey were placed in separate foster homes. Peter was brought up by parents oflow intelligence with poor ____(education) opportunities. Mark was educated inthe home of well­to­do parents ____ had been to college. This environmentaldifference continued until the twins were in their late teens, when they weregiven tests to measure their intelligence. Mark's I．Q. was 125, twenty­five points higherthan the average and fully forty points higher than his twin brother.