完形填空

阅读下面短文，掌握其大意，然后从以下各题所给的四个选项(A、B、C和D)中，选出最佳选项。

　　For some time past it has been widely accepted that babies learn to do things because certain acts lead to “rewards”(报偿); and there is no reason to 1 that this is true. But it used also to be 2 believed that effective rewards, at least in the early stages, had to be directly connected to such basic physiological (生理的)“drives”as thirst or 3 .In other words, a baby would learn 4 he got food or drink or some sort of physical comfort, not otherwise.

　　It is now clear that this is not so. Babies will learn to act in ways that 5 results in the world with no reward except the successful result.

　　Papousek began his studies by using 6 in the normal way to “reward” the babies and so 7 them to carry out some 8 movements, such as turning the 9 to one side or the other. Then he noticed that a baby who had had enough to 10 would refuse the milk but would 11 go on making the learned response (反应) with clear 12 of pleasure. So he began to 13 the children's responses in 14 where no milk was provided. He quickly found that children as young as four months would learn to turn their heads to right or left if the movement “turned on” 15 and indeed that they were able to learn quite complex(复杂的)turns to bring about this 16 , for instance, two left or two right, or 17 to make as many as three turns to one side. Sometimes they would not turn back to watch the lights closely.

　　Papousek 18 that it was rat mainly the sight of the lights that 19 them, it was the success they were achieving in 20 the problem, and that there exists a basic desire to understand and control the world.

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