在选择题中,同时有a friend of my father 和 a friend of my father's 选什么?

在选择题中,同时有a friend of my father 和 a friend of my father's 选什么?
比如：
----Doyou know the man who is playing tennis there?
----Certainly.He is a friend of _______.
A.my father's
B.my father
我觉得A和B，都说得通，但答案为A
求解释