The new system needs to be in operation for five months it can be updated.A. asB. whileC

问题补充：

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：

A. as

B. while

(答案→)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。