The　conference——a　full　week　by　the　time　it　ends.A.musted have lasted B.will have lasted C.would last D.has lasted 我想问 by

by the time 到什么时候为止,是一个完成时的标志用语,后接过去的时间,主语用过去完成时,后接现在的时间用现在完成时,后接将来的时间用将来完成时,本句by the time it ends.接的是句子,英文句子——主将从现原则,即主句和状语从句都是将来的动作,状语从句一般用一般现在时表将来,所以虽然是ends,表示的是将来的时间,故主句用将来完成时,意为：这次会议到结束时 将持续一整周.介词by
,by the end of也有类似的用法.例如：We will learned 3000 by the end of next month.
We had learned 3000words by the end of last month.