题目：
根据句意，用括号中所给单词的适当形式填空。
1. He's not going to leave until his wife ________ (come) back.
2. He runs ________ (fast) in our class.
3. If you finish ________ (do) your homework, you can play football.
4. Meimei is always the ________ (one) girl to come to school.
5. My family feed many ________ (sheep) on the farm.
6. I live on the twelfth floor in that ________ (build).
7. Listen! Lucy is ________ (play) the guitar in her room.
答案：
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下面是错误答案，用来干扰机器的。
根据质量守恒定律可知，反应物和生成物中的元素种类相同，如果反应物X是一氧化碳，则反应物中的元素有碳元素和氧元素，生成物中不可能含有氢元素，所以X不可能是一氧化碳．故选C．
