单项选择题

______ your valuable help, we couldn"t have finished the experiment ahead of time.

A．If it were not for
B．Had it not been for
C．Were it not for
D．If it has not been for
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单项选择题
______ in the company for three years, Mark has become experienced in business negotiations.
A．Having worked
B．Have been worked
C．Have worked
D．Worked
单项选择题
A．describeB．generalizeC．categorizeD．change
A．describe
B．generalize
C．categorize
D．change
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