a friend of her还是a friend of hers

a friend of her还是a friend of hers 请问,为什么是a friend of hers 而不是a friend of her? Judy is a friend of (选择） A me B her C she D hers Betty met a friend of (she,her ,hers)in the street. A friend of hers came to see you last night. a friend of my还是a friend of mine a friend of Jack还是 a friend of Jack's a friend of I还是a friend of me Friend of a Friend 歌词 Helen is a friend of___.A.hers B.her a friend of mine. a friend of mine