单项选择题

Experts say walking is one of the best ways for a person to ______ healthy.

A．preserve
B．stay
C．maintain
D．reserve
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单项选择题
The professor can hardly find sufficient grounds ______ his argument in favor of the new theory.
A．which to base on
B．on which to base
C．to base on which
D．which to be based on
单项选择题
Even when textbooks are ______ through a school system, methods of teaching may vary greatly.
A．commonplace
B．standardized
C．competitive
D．generalized
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