【题目】He tried his best to solve the problem, __________ difficult it was.
A.however
B.no matter
C.whatever
D.although

【答案】A
【解析】A. however无论如何....；B. no matter无论；C. whatever连词，无论什么；D. although尽管；句意：无论它是多么的难，他都尽力解决这个问题。根据句意，故选A。