问题补充:
By the time he arrived at the airport, the plane .
A. has taken off
B. was taking off
C. would take off
D. had taken off
答案:
A. has taken off
B. was taking off
C. would take off
(答案→)D. had taken off
解析:考察时态。句意:当他们到达机场时,飞机已经起飞了。可知叙述的是过去某个时间已发生的动作,故用过去完成时态。选D。