He tried his best to solve the problem difficult it was.A. no matterB. no matter howC. wh

问题补充：

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

A. no matter

B. no matter how

C. whatever

D. although

答案：

A. no matter

(答案→)B. no matter how

C. whatever

D. although

解析：考查让步状语从句。No matter +特殊疑问词引导让步状语从句，意为：无论…；意为后面接形容词difficult，所以使用how.而不是what.句意：无论问题有多难，他都全力以赴地解决它。故B正确。