完形填空

阅读下面短文，掌握其大意，然后从每小题所给四个选项中，选出一个最佳答案。

　　Do you know how our brains work?

　　During the 19th century scientists found that when 1 parts of the brain of a man were 2 , he would lose the 3 to do certain things. And so people 4 that each of the brain does a different 5 . But modern people research has found out that this is not so, for it is not easy to say 6 what each part of the brain does.

　　In the 7 fifty years there has been a great increase in the amount of research 8 on the brain. Chemists and biologists have 9 that the way the brain works is not so 10 as people in general may think. 11 tell us that 100, 000 chemical changes take place in the brain every, second. Some recent research also 12 that we can remember everything that happens to us. We may not be able to recall the things we've heard and seen, 13 it is all kept there in the storehouse of the human being.

　　Earlier scientists thought the power of one's brain got weaker and weaker as one grew 14 . But it is now thought that it is not 15 . As long as the brain. is 16 plenty of exercise it keeps its ability. It has been 17 that an old person who has always been active in the mind has a 18 mind than a young person who has only done 19 work without using much of his brain. It is now 20 that the more work our brains work, the more work they are able to do.

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