请教几道选择题 ，谢谢！
请教各位老师，同学几道题，是从网上下的题 ，没有答案，不知道做的对不对，本人能力有限，可能错误很多，请大家指正，望朋友别见笑，诚心求解，谢谢！
21. I had just started back for the house to change my clothes __ I heard voices.
过去完成时
A as B while C after D when
22. I felt somewhat disappointed and was about to leave, __ something occurred which attracted my attention.
A unless B until C when D while
23. It was essential that these application forms __ back as early as possible.
A must be sent B will be sent C are sent D be sent
24. The children went there to watch the iron tower __.
A to erect B be erected C erecting D being erected
25. The engine __ smoke and steam.
A gives up B gives in C gives away D gives off
26. The manager promised to keep me __ of how our business was going on.
A to be informed B on informing C informed D informing
27. The goals __ he had fought all his life no longer seemed important to him.
A after which B for which C with which D at which
28. I should like to rent a house, modern, comfortable and __ in a quiet neighborhood.
A all in all B above all C after all D over all
29. __ we have finished the course, we shall start doing more revision work.
A For now B Now that C Ever since D By now
30. What you have done is __ the doctor’s orders.
A attached to B responsible to C resistant to D contrary to
本人答案：DBADD DBBDC
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亲们啊，怎么没人回贴啊~急求中
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因为我素语法白痴。。。
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因为我素语法白痴。。。
