however作連接副詞時，與no matter how相當，後接形容詞或副詞

例:

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

A. however B. no matter C. whatever D. although 
.

.

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【分析】答案選A。

因difficult是形容詞，修飾形容詞要用連接副詞however(無論如何，不管多麼)，引導一個讓步狀語從句。