english 问题请教各位大哥

english 问题请教各位大哥
Miss Green is a friend of _____.
A Mary's mother's B Mary's mother C Mary mother's D mother's of mary
这是我做的一道题 答案选A
但今天我看到一道题
-----Who's ______boy in red ,do you know?
-----Oh.He's _____friend of Tom
A the ;a B an; the
C a; the C the;an
答案选A
我要问的问题是为什么第一题是Miss Green is a friend of Mary's mother's.
第二题却是He's a friend of Tom
一个后面有's 一个就没有
或者是否题出错了？？？