问题
选择题
He tried his best to solve the problem,________ difficult it was.
A．however
B．no matter
C．whatever
D．although
答案
A
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。