The factory produces two million pairs of shoes every year, half are sold abroad.A. of w

The factory produces two million pairs of shoes every year, half are sold abroad.
A. of which
B. which of
C. of them
D. of that

(答案→)A. of whichB. which ofC. of themD. of that解析： 本题考查定语从句。“逗号”不能连接两个句子，所以所选之词应是连词，排除C项；that不能引导非限制性定语从句，排除D；介词应用在of前。

就是这个解释