请简单描述SEO和SEM的区别，至少写出5个方面的区别。

A. able to be used
B. to step down from a position well liked
C. to do well

A. a lot
B. to legally take a child of other parents as one’s own
C. to create something that didn’t exist before

A. able to be used
B. an area of study or activity
C. a school term or period of time