The new system needs to be in operation for five months it can be updated.A．as B．whileC．before D．once 题目和参考答案——青夏教育精英家教网—

题目内容

The new system needs to be in operation for five months ________ it can be updated.

A．as B．while

C．before D．once

【解析】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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