1.He hasn't heard from his friend _____last month.

1.He hasn't heard from his friend _____last month.
A.since B.by the end C.for D.until
2.Don't hurry.The bus won't start_____everybody gets on.
A.since B.as C.until D.when
3.Please show me_____to send an e-mail,John.It's the first time for me to do it.
A.how B.what C.when D.where
要有详细的原因!