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

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(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。