单项选择题

According to the passage, virgin-soil epidemics can be distinguished from other catastrophic outbreaks of diseases in that virgin-soil epidemics ______.

A．recur more frequently than chronic diseases
B．involve populations with no prior exposure to a disease
C．usually involve a number of interacting diseases
D．are less responsive to medical treatment than are other diseases
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单项选择题
It can be inferred from the text about the Native American inhabitants of Ungava Bay that they ______.
A．were immunologically defenseless against measles
B．could scarcely ward off the fatal attack by the 1952 epidemic
C．were the last native people to be struck by a virgin-soil epidemic
D．didn't come into frequent contact with White Americans until the 20th century
单项选择题
No one can expect to fully enjoy the adulthood privileges until be is ______.
A．eleven years old
B．sixteen years old
C．twenty-one years old
D．between twelve and twenty-one years old
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