He tried his best to solve the problem difficult it was.A. howeverB. no matterC. whatever

问题补充：

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

A. however

B. no matter

C. whatever

D. although

答案：

(答案→)A. however

B. no matter

C. whatever

D. although

解析：however+adj./adv.引导让步状语从句，意为“无论多么”。no matter不能单独使用，后须加what/how/when/where等连词。C、D两项后不能接形容词，故答案为A。