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

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：考察让步状语从句。本题中however引导的是让步状语从句，although引导的让步状语从句中使用陈述语序，whatever后面要接名词或者名词短语。No matter 后面要接特殊疑问词，来引导让步状语从句。本句中横线后面是形容词difficult，故使用however来修饰。句意：无论多么困难，他都会全力以赴地解决这个问题。故A正确。