Yesterday we called at----.A.a friend of my uncle's ;B.uncle

Yesterday we called at____.A.a friend of my uncle's ;B.uncle's; C.my uncle ;D.my uncle's
这道题为什么选择A呢?虽然called是非谓语动词,但是my uncle 不也行么?跟被动语态和双重所有格有关么?急.