Is the backpack on the desk? .A．No, it is.B．Yes, it isn’t.C．Yes, it is.D．No, it’s no. 题目和参考答案——青夏教育精英家教网—

       Dogs are friends of man. They work for us. They hunt (狩猎) with us. They play with us. But once,
all over the world, dogs are wild (野生的).
       Dogs go back to the Stoen Age. All dogs had the same ancestor(祖先). It is believed that this
ancestor was much like a wolf (狼). Other animals like the fox, came from this ancestor, too.
       Thousands of years ago, man began to tame (驯服)the wild dogs . When the dogs were tame, they
were trained (训练). The strong dogs became working animals . They were trained to pull heavy loads
(担子). They learned to keep watch over the sheep and other animals. Working dogs had other jobs, too.
       Some dogs were not strong. But they could help man hunt for game. Other dogs were best as pets.
       At first , there were only a few kinds of dogs. Today there are more than 100 kinds.

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