1.Mr.Smith has _____ a bank clerk for five years.

1.Mr.Smith has _____ a bank clerk for five years.
A.be B.got C.become D.been
2.I think the best way _____ learn English is to use it as often as possible.
A.for B .of C.on D .to
3.They have been friends ______ they met at school
A.when B.until C.since D.before
4.我已经决定要离开了.
I have _____ _____ leave.
最好说下理由,为什么是这个