David knocked at the door for nearly half an hourMary opened it unwilling.A. whenB. while

问题补充：

David knocked at the door for nearly half an hourMary opened it, unwilling.

A. when

B. while

C. until

D. before

答案：

A. when

B. while

C. until

(答案→)D. before

解析：考查状语从句中的连词。句意：David敲门敲了将近半个小时Mary才不情愿地打开。状语从句中表示“过多久…才…”为before 的用法。选D。