A man does not know the difficulty of anything _____ he does it personally .
A.although
B.if
C.because
D.unless
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D解析连词unless本身具有否定意义：if … not，引导的是否定的条件句，这种否定的条件句从反面来表达强烈的语气，一些语法学家称它为反面的唯一条件句又如：One cannot master a foreign language well unless he studies it hard . (不下苦功夫，是学不好的一门外国语的。)