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完形填空
Number sense is not the ability to count．It is the ability to recognize a 1 in number．Human beings are born with this ability． 2 , experiments show that many animals are, too．For example, many birds have good number sense．If a nest has four eggs and you remove one, the bird will not 3 ．However, if you remove two, the bird 4 leaves．This means that the bird knows the 5 between two and three．
Another interesting experiment showed a bird's 6 number sense．A man was trying to take a photo of a crow(烏鴉)that had a nest in a tower, but the crow always left when she saw him coming．The bird did not 7 until the man left the tower．The man had an 8 ．He took another man with him to the tower．One man left and the other stayed, but they did not 9 the bird．The crow stayed away until the second man left, too．The experiment was 10 with three men and then with four men．But the crow did not return to the nest until all the men were 11 ．It was not until five men went into the tower and only four left that they were 12 able to fool the crow．
How good is a human's number sense? It's not very good．For example, babies about fourteen months old almost always notice if something is taken away from a 13 group．But when the number goes beyond three or four, the children are 14 fooled．
It seems that number sense is something we have in common with many animals in this world, and that our human 15 is not much better than a crow's．
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