单项选择题

According to the passage,______.

A．more and more women are infected by HIV virus
B．women have more chances of being infected by HIV virus
C．the aim of the study in this passage is to understand how the virus works
D．many medical studies didn't answer any questions

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单项选择题

It will be even more difficult to develop a vaccine because______.

A．most women are infected by a single variant of the HIV virus
B．the virus can duplicate itself into more genetic variants
C．it is difficult to monitor the activities of the viruses
D．technologies are not advanced now