谁能帮我解析几道英语习题

谁能帮我解析几道英语习题
1.Tom had opened the door _____ mary had time to knock.
A.until B.before C.as D.although
2.It's high time you ______.
A.do you homework B.would have done your homework
C.had done your homework D.did your homework
3.The bus wsa held up on way to the airport,otherwise I_____the plane.
A.must catch B.have caught C.could have caught D.ought to catch
4.Let's ____when the train will arrive.
A.discover B.find C.find out D.look
5.If this continues to tske place at the present rate,____another fifty years,the world will be crowded with big ants.
A.at B.for C.in D.on
6.john will ___ Jack to be the general manager,for Jack is goning to channge his work.
A.take place B.take the place of C.take his place D.take the place