单项选择题

The investigation group tried in ______ to discover what caused the failure of the engine.

A．diligence
B．vain
C．vigor
D．zeal
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单项选择题
A soldier has to learn to carry ______ orders as soon as they are given.
A．on
B．out
C．through
D．off
单项选择题
I didn't ______ the meeting because I was ill.
A．join
B．enter
C．attend
D．appear
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