Human beings act in a different way from that of animals just because they can speak while

Human beings act in a different way from that of animals just because they can speak while animals cannot. Even the cleverest animals cannot do things which to us seem very simple and which small children, as soon as they learn to talk, would be able to do. A German scientist, who made experiments for many years with big apes, found that his apes could use sticks as tools to pull down bananas which they could not reach. But they only used the stick to get a banana when both the stick and the banana were in view at the same time. If the banana was in front of them and the stick was behind them, they could not use the stick. They could not bear the banana in mind long enough to look around and pick up the stick and then use it. The reason for this is clear. We have words for banana and stick which help us to think about these things when they are not in sight. Even a small child knowing the words "banana" and "stick" has an idea of their relationship and is able to think of "stick" together with "banana" and to remember this long enough to pick the stick from behind and use it on the banana. Unable to speak, animals cannot keep their knowledge of things for long. That is why they often interrupt one line of action to something else and later forget it completely. Human beings, on the other hand, use language and are able to go after one thing continuously and do it with a definite aim.

Human beings and animals act very differently because______.

A．human beings know how to use a stick while animals don"t know

B．human beings can think while animals can"t

C．human beings have a good memory while animals haven"t

D．human beings can speak while animals can"t

请帮忙给出正确答案和分析，谢谢！