题目

下列说法中不正确的是（ ）A.催化剂能改变化学反应速率，但不增加生成物的质量B.能产生温室效应的气体有二氧化碳、臭氧、甲烷等C.氧气的化学性质很活泼，能与所有物质发生化学反应D.空气里氮气、氧气等分子均匀地混合在一起 答案：【答案】C【解析】A、催化剂能改变化学反应速率，但不会增加生成物的质量，故选项说法正确。B、能产生温室效应的气体有二氧化碳、臭氧、甲烷等，故选项说法正确。C、氧气的化学性质比较活泼，能与很多物质发生化学反应，故选项说法错误。D、空气是一种混合物，空气中氮气、氧气等分子均匀 完形填空(20分) 　　How are letters delivered in your country？Are letters delivered to 　　1　　 person's door？Is mail 　　2　　 at the post office until someone goes there to get it？ 　　3　　 someone lives far from a post office，how does he get his mail？ 　　In the United States，most people who live in cities and small towns 　　4　　 their mail at home．Letters are delivered to the family's door every day．Some people have boxes or baskets for letters beside the front door．If there is no box or basket，the letters are 　　5　　 an opening in the door．Large packages(包裹)are sometimes 　　6　　 beside the door． 　　Many Americans live on farms that are far from 　　7　　 city，town，or village．Their mail is delivered to 　　8　　 near their houses．Every day the farmers who live near the road 　　9　　 to their mailboxes．This mail service is called Rural(农村的)Free Delivery．Rural Free Delivery is also 　　10　　 as RFD． (1) [　　] (2) [　　] (3) [　　] (4) [　　] (5) [　　] (6) [　　] (7) [　　] (8) [　　] (9) [　　] (10) [　　]