He tried his best to solve the problem difficult it was.A. whateverB. howeverC. whicheve

问题补充：

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

A. whatever

B. however

C. whichever

D. wherever

答案：

A. whatever

(答案→)B. however

C. whichever

D. wherever

解析：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。