either of them 和 each of them

问题描述:

either of them 和 each of them
The old woman has two grandsons.____ middle school student.
A each of them is a B either of them is a C both of them are D they are both 正确答案是B 但我不懂

这个题根据前一句话的意思,这老太太有两个孙子.其中有一个是中学生.
两者其中之一用either.each 是用于三者或者三折以上.
both表两者都是.如果用这个后面应该是负数.students