more pretty than 和 pretter than 区别?

more pretty than 和 pretter than 区别?
看这两个选择,
1 although she is very popular,she is not ____ her sister
a prettier than
b more pretty than
2 they are _____ my other neighbors
a friendlier than
b more friendlier than
c more friendly than
为什么第一题就选A第2题就选C呢?
more pretty than
pretter than
more pretter than
这三个区别是什么?最后一个是不是就不对?没有这样的用法?