【题目】The factory produces half a million pairs of shoes every year, _______80% are sold abroad.
A.of which
B.which of
C.of them
D.of whom

【答案】A
【解析】句意：这家工厂每年生产五十万双鞋，其中80%都被销售到了国外。由结构判断后面是非限制性定语从句，排除C选项，而B选项形式错误，D项whom指人，应该是指物用which，所以答案是A。