I think that she must be a relation of ____ .

I think that she must be a relation of ____ .
A、theirs
B、them
C、their
D、they’s
【正确答案】：A
【题目解析】：theirs 名词性物主代词，他们的；句意为：我想她一定是他们的亲戚。这里要使用名词性物主代词，而their 形容词性物主代词，后面要接名词。