单项选择题

It is of vital importance that a person ______ at least both physically and mentally strong to be an astronaut(宇航员).

A．shall be
B．be
C．are to be
D．must be
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单项选择题
In the advanced reading course students must take tests at monthly ______.
A．distance
B．length
C．periods
D．intervals
单项选择题
Make sure to ______ the children ______ swimming in the dirty river.
A．protect, from
B．keep, from
C．avoid, from
D．ask, from
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