The watch was given by ___ for her birthday.填 a friend of my sister's

问题描述:

The watch was given by ___ for her birthday.填 a friend of my sister's

对为什么需要's呢?不是前面已经有OF了吗?看汉语意思似乎可以不用's,但是英语就是要写全a friend of my sister's 其实是a friend of my sister'sfriends省略了friends