单项选择题

The biggest worry that the researchers now have is ______.

A．more and more wild chimpanzees are being slaughtered
B．AIDS virus is difficult to be killed and controlled
C．it is not easy to repair people's deficient immune systems
D．many dangerous viruses are being transmitted to humans
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单项选择题
The reason some children are backward in speaking is most probably that ______.
A．they are incapable of learning language rapidly
B．they are exposed to too much language of once
C．their mothers respond inadequately to their attempts to speak
D．their mothers are not intelligent enough to help them
单项选择题
Which of the following can NOT be inferred from the passage
A．The faculty of speech is reborn in man.
B．Encouragement is anything but essential to a child in language learning.
C．The child's brain is highly selective.
D．Most children learn their language in definite stages.
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