如图所示，比较他们运动的快慢，物理学中我们通常可以采用相同____ 比较____ 的方法。

    There are many different kinds of parrots in the world. They can not only(不但)live in the tree, but they can also(而且)live on the mountain. Some parrots are quite big. Some are very small. Many people like them, because they often have beautiful colours, and because they can “talk”.

    Do you know why parrots can talk? But most birds cannot. Perhaps, parrots can “talk” because they have big, thick tongues. But some other birds that cannot talk have big, thick tongues, too. So we don't know why parrots can talk.

下列计算中，结果正确的是（   ）