请问这道题选那个答案呢?为什么?

D
此处应该是将来完成进行时,因为by next summer指到下一个夏天为止
构成：主语+ shall/will have been doing
将来完成进行时表示动作从某一时间开始一直延续到将来某一时间.是否继续下去,要视上下文而定.这个时态常和表示将来某一时间的状语连用.
如:I shall have been working here in this factory for twenty years by the end of the year.到今年年底,我将在这个工厂工作20年了
If we don't hurry up the store will have been closing before we get there.咱们如不快一点儿,等我们到了那儿,店门就会关了.
The play is coming off in August.By then the play will have been running for three months.这个剧将于8月停演.到那时为止,这个剧将连演三个月了.