For Young People, How Does a Cohabiting Relationship Usually Begin

For young people, how does a cohabiting relationship usually begin?

A) Most young people who live together decided to do so after becoming engaged.
B) Most young people decide to move in together after calculated planning.
C) Most young people come to live together by drifting into it.
D) Most young people start living together after one person is forced out of a parent's house.