21.—I wonder how much you charge for your services.
—The first two are free ________ the third costs $30.
A.while B.until
C.when D.before
答案：A 考查连词的用法。while表对比，意为“而”。
22. How can they expect to learn anything ________ they never listen?
A.whether B.when
C.but D.unless
23. She usually keeps her valuables, such as jewels and money, ________ she believes secure.
A.even if B.in case
C.where D.that
答案：C 考查状语从句。where引导的地点状语从句。句意为：她总是把贵重的东西，比如首饰、钱等放在她认为安全的地方。
24.Please call my secretary to arrange a meeting this afternoon, or ________ it is convenient to you.
A.whenever B.however
C.whichever D.wherever
25.——Did you inform Helen of the plan put forward at the meeting?
—Of course. ________ I saw her.
A.So long as B.If only
C.The moment D.Any time
26. Scientists are warning that ________ emissions of carbon (碳排放) are cut, we could be heading for an environmental disaster.
A.before B.as
C. unless D.in case
27. I will accept Guy coming with us, ________ he pays for his own meals.
A.only if B.so that
C.ever since D.even though
答案：A 考查条件状语从句。分析题干的逻辑关系可知，后句是前句的条件，所以用only if。句意：只要他自己付钱吃饭，我就接受Guy和我们一起去。
28. ________ H7N9 is a deadly virus, it is still safe to eat thoroughlycooked chicken.
A.Until B.While
C.If D.Since
29. The movie Life of Pi is so attractive that it holds the audience's curiosity ________ the story reaches the end.
A.until B.before
C.after D.when
30. Jerry has promised to keep the secret, so he won't tell anyone ________ asked to.
A.until B.now that
C.unless D.even though
31. It will be another 50 years ______ our country is expected to reach the level of a medium developed country.
A.since B.before
C.that D.when
32. House prices vary from place to place and are usually high ________ there are famous schools.
A.when B.since
C.where D.whether
33. We can't wait any longer ________ we'll miss the plane.
A.or B.but
C.so D.yet
34. The new system needs to be in operation for five months ________ it can be updated.
A.as B.while
C.before D.once
35. ________ it is, the boy never seems to be able to finish it off.
A.However interesting a book
B.Whatever interesting a book
C.How interesting a book
D.What an interesting book
答案：A 考查特殊句式和状语从句。句意为：不管这本书多么有趣，那个孩子看起来是绝不会把它看完的。感叹句的词语排列顺序为：how+形容词+冠词+可数名词单数=what+冠词+形容词+可数名词单数。根据句意可知，此处表示“不论到什么程度，不管”，故应用however，故答案A符合语境。
36. Steve was busy clearing the cups and dishes from the table ________ others were playing.
A.until B.if
C.since D.while
37.You seem to have a readymade answer ________ I ask you a question.
A.whenever B.whatever
C.whichever D.whoever
38.—I don't mind how you do it ______ you finish the painting on time.
—OK, I see.
A.as well as B.as far as
C.as long as D.as fast as
39.—Was it 11 o'clock ________ your father came back last night?
—Yes, he is always coming back so late.
A.that B.while
C.when D.until
40. ________ you need something, please don't hesitate to let me know.
A.Even though B.In case
C.As though D.Even if
