By the time Pierce es back from the meeting，we here for three hours.
A. will be waiting
B. is waiting
C. will wait
D. will have waited
网友回答
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。