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。
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