A man does not know the difficulty of anything he does it personally .
A. although
B. if
C. because
D. unless
网友回答
A. although
B. if
C. because
(答案→)D. unless
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。)