阅读下面短文，从短文后各题所给的四个选项(A、B、C和D)中，选出可以填入空白处的最佳选项。

    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 not3. However, if you remove two, the bird4leaves. This means that the bird knows the5   between two and three.

    Another interesting experiment showed a bird's6number sense. A man was trying to take a photo of a crow(乌鸦)that had a nest in a tower, but the crow always left when she saw him coming. The bird did not7until the man left the tower. The man had an8.He took another man with him to the tower. One man left and the other stayed, but they did not9 the bird. The crow stayed away until the second man left, too. The experiment was10with three men and then with four men. But the crow did not return to the nest until all the men were11.It was not until five men went into the tower and only four left that they were12 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 are14fooled.

    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.