Number sense is not the ability to count. It is the ability to recognize a __16__ in number. Human beings are born with this ability. __17__, 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 __18__. However, if you remove two, the bird __19__ leaves. This means that the bird knows the __20__ between two and three.
Another interesting experiment showed a bird's __21__ number sense. A man was trying to take a photo of a crow that had a nest in a tower, but the crow always left when she saw him coming. The bird did not _22__ until the man left the tower. The man had an __23__. He took another man with him to the tower. One man left and the other stayed, but they did not __24__ the bird. The crow stayed away until the second man left, too. The experiment was __25__ with three men and then with four men. But the crow did not return to the nest until all the men were __26__. It was not until five men went into the tower and only four left that they were __27__ 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 __28_ group. But when the number goes beyond three or four, the children are __29__ fooled.
It seems that number sense is something we have in common with many animals in this world, and that our human __30__ is not much better than a crow's.
空白处选择
A.rise
B.pattern
C.change
D.trend