单项选择题

Frequent exposure to X-ray is harmful, so the use of X-ray should be limited to occasions when________.

A．primary
B．elementary
C．necessary
D．emergent
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单项选择题
I don't want to work here so I _______ my resignation yesterday.
A．substituted
B．submitted
C．subjected
D．submerged
单项选择题
With all its advantages, the computer is by no means without its _______.
A．boundaries
B．confinements
C．restraints
D．limitations
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