题目内容
27. You should _______ some steps to help maintain a healthy indoor air environment.
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28. Believe it or not, you can find many ways to save money while ________ in an expensive area.
29. We went to the museum only ________ it was closed that day.
30. If you want to buy these new devices, you should be clear about ________ they can offer.
31. We had hardly entered the office building ________ the fire alarm went off.
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