He met many problems---he was going over his lessons.A. beforeB. as soon asC. sinceD.

问题补充：

He met many problems------ he was going over his lessons.

A. before

B. as soon as

C. since

D. while

答案：

A. before

B. as soon as

C. since

(答案→)D. while

解析：考查连词。此句中主句中是一般过去时，而从句中是过去进行时，故用while。