difficult the problem was he tried his best to solve it.A. No matter B. Although C. What

问题补充：

difficult the problem was, he tried his best to solve it.

A. No matter

B. Although

C. Whatever

D. However

答案：

A. No matter

B. Although

C. Whatever

(答案→)D. However

解析：本题考查状语从句。however=no matter how不管多么难。句意为：不管这个问题有多难，他都尽力去解决。