become的过去式和过去分词怎么写

　　The environment has become a very hot issue.

　　环境已成为很热门的话题。

　　He became very weak towards the end of his life.

　　他临终时很虚弱。

　　What became of that student who used to live with you?

　　以前和你住在一起的那个学生后来怎么样了？

　　My parents had fixed ideas about what I should become.

　　父母对我应该成为什么样的人有定见。