– Why did they help Mrs Brown with her work?– Because she was .A. an old friend of themB.

问题补充：

– Why did they help Mrs Brown with her work?

– Because she was .

A. an old friend of them

B. the friend of them

C. on the their friend

D. a good friend of theirs

答案：

A. an old friend of them

B. the friend of them

C. on the their friend

(答案→)D. a good friend of theirs

解析：句意：他为什么帮助布朗夫人做她的工作？——因为她是他们的一个好朋友。Of在表示所有关系时，后面接名词，them是人称代词。选项ABC结构有误，故选D，名词性物主代词做介词宾语。