单项选择题

The work to stamp out smallpox was made more difficult by______.

A．people's unwillingness to report cases
B．the lack of vaccine
C．the backwardness of the region
D．the incessant local wars
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单项选择题
Which of the following conclusions about mammoths does the passage support
A．Humans hunted them to extinction.
B．The freezing temperatures of the Ice Age destroyed their food supply.
C．The cause of their extinction is not definitely known.
D．Competition with mastodons caused them to become extinct.
单项选择题
The volunteers mentioned were paid to______.
A．find out about the reported cases of smallpox
B．vaccinate people in remote areas
C．teach people how to treat smallpox
D．prevent infected people from moving around
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