He tried his best to solve the problem, difficult it was.
A. no matter
B. no matter how
C. whatever
D. although
网友回答
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。