问题
选择题
A man does not know the difficulty of anything_________ he does it personally.
A．although
B．if
C．because
D．unless
答案
答案：D
连词unless本身具有否定意义：i￡．．not，引导的是否定的条件句，这种否定的条件句从反面来强调的语气，一些语法学家称它为“反面的惟一条件句”，如： One cannot master a foreign language well unless he studies it hard．