单项选择题

A considerable amount of time and money has been invested in ______ this system.

A. defining
B. implying
C. reducing
D. perfecting
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单项选择题
The teachers tried to （） these students that they could solve the complicated problem, however, they just didn't see the point．

A．convince
B．encourage
C．consult
D．concern
单项选择题
Americans are highly ______, and therefore may find it difficult to become deeply involved with others.
A. moving
B. mobile
C. movable
D. motional
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