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
Many AIDS experts are not completely satisfied with results of the study because _______.
A．only a limited number of chimpanzees are used for sampling the virus
B．it is now extremely difficult to find chimpanzees that carry the virus
C．the samples collected are from two different subspecies of chimpanzees
D．it does not provide reliable evidence of the link between SIV and HIV-I
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