Human beings act in a different

Human beings act in a different way from 21______ of animals just because they can speak while animals cannot.22______ the cleverest animals cannot do things which to us seem very 23______ and which small children,24______ they learn to talk, would be able to do.
A German scientist, who 25______ experiments for many years with big apes, found that his apes could 26______ his sticks as tools to pull down bananas which they could not 27______. But they only used the stick to get a banana when both the banana and the stick were 28______ 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 29______ the banana 30______ enough in mind to look around and then pick up the stick and use it.
29.
A、maintain
B、keep
C、remain
D、insert
【正确答案】：B