题目

Number sense is not the ability to count. It is the ability to recognize a    14    in number. Human beings are born with this ability.  15  , 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   16  . However, if you remove two, the bird  17  leaves. This means that the bird knows the   18   between two and three. Another interesting experiment showed a bird's  19  number sense. A man was trying to take a photo of a crow(乌鸦)that had a nest in a tower, but the 倡导“免赠贺卡”“免用一次性筷子”的出发点是 [　　] A．减少个人经济开支 B．节约木材，保护森林 C．减少固体垃圾 D．移风易俗