问题补充：
My friend’s flight had taken off I got to the airport to see him off.
A. while
B. after
C. before
D. until
答案：
A. while
B. after
(答案→)C. before
D. until
解析：
试题分析: 句意：在我到达机场送朋友前，他的航班已经起飞了。while当……的时候， after在……之后， before在……之前，until直到……。take off起飞，see off送行。got to一般过去时，had taken off过去完成时，可知主语动作发生在从句之前，故选C.
