By the time Pierce comes back from the meeting wehere for three hours.A. will be waitingB.

问题补充：

By the time Pierce comes back from the meeting，wehere for three hours.

A. will be waiting

B. is waiting

C. will wait

D. will have waited

答案：

A. will be waiting

B. is waiting

C. will wait

(答案→)D. will have waited

解析：

考查时态。by the time 引导的时间状语从句的主句一般用将来完成时。