17. C　18. B

　　19. D　解析：由but if not managed可知，动机会让我们“受益(benefit)”，但如果掌控不好，它也会让我们“脱离(lose touch with)”现实并导致一些“消极的(negative)”行为。

　　20. D　解析：由上文的we should learn to manage out motivation可知，这样我们才会在生活和工作中更“有效率(effective)”。

　　.短文改错

　　Recently the issue of students' study has brought to public attention. Students are anxious about what to improve their scores. Some work very hard. They spend all their time in study. They do nothing except to study. As far as I'm concerned, it is not a good way to study. Alternating work with rest was the best way to study well. Students should try their best to learn knowledges as Francis Bacon said, “Knowledge is power.” Therefore, students should have other recreational activities to relax themselves, because people can do things more efficient after relaxing themselves. What's more, people can obtain knowledge from many sources besides books, that is beneficial for study. Thus, combining exertion and rest are the best to study.

　　答案：

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