John waited at the bus-stop for nearly half an hour the bus finally arrived .A. whenB. as

问题补充：

John waited at the bus-stop for nearly half an hour the bus finally arrived .

A. when

B. as

C. before

D. while

答案：

A. when

B. as

(答案→)C. before

D. while

解析：before引导时间状语从句，公共汽车来之前，我已等了将近半个小时了。故选C。