. As a teacher he has never given his students so difficult a problem they can’t work ou

问题补充：

. As a teacher, he has never given his students so difficult a problem they can’t work out.

A. if

B. that

C. when

D. As

答案：

A. if

B. that

C. when

(答案→)D. As

解析：可联系答.案.网客服索取。