问题
选择题
The new system needs to be in operation for five months ________ it can be updated.
A．as
B．while
C．before
D．once
答案
答案：C
C考察状语从句。本句的before用语肯定句中，强调主句动作持续的时间比较长，意为：要多多久….才。故C正确。
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The new system needs to be in operation for five months ________ it can be updated.
A．as
B．while
C．before
D．once
答案：C
C考察状语从句。本句的before用语肯定句中，强调主句动作持续的时间比较长，意为：要多多久….才。故C正确。