快高考了.It is said that a polar winter,_____,will stay for more than six month.

题目

快高考了.It is said that a polar winter,_____,will stay for more than six month.
A.once arrives B.once arrived C.once it arrives D.it once arrives
为什么A不行..

答案

Once是连词,在这里引导的条件状语从句作插入语.句意是“据说极地的冬天一旦到来,将会持续六个月以上”.once和if,when等用法差不多,后面引导的从句若主语和主句的主语一致的时候,可以省略主语（你估计是这一点有点糊...