题目

氧元素是地壳中含量最多的元素。下列说法中正确的是( ) A．16O2与18O3互为同位素 B．氧元素与其他短周期非金属元素均能形成酸性氧化物 C．氧与钠形成的稳定化合物中，阴、阳离子个数比均为1∶2 D．O2变成O3以及16O变成18O的变化均属于化学变化 答案：解析：选C 同位素是原子之间的关系，故A错；氧元素与氢元素形成的H2O、H2O2均不是酸性氧化物，故B错；Na2O和Na2O2(其阴离子为O)中，阴、阳离子个数比均为1∶2，故C正确；O2变成O3属于化学变化，16O变成18O不属于化学变化，故D错。Tim Becker and his neighbors are doing something to make their neighborhood a trouble-free area. When Tim Becker gets in his car to go shopping, he doesn't　　1　　drive to a store and back home. He always looks　　2 　 up and down the streets of his neighborhood. He looks for anything　　3　 , such as strange cars, loud noises,　　4 　 windows, or people gathering on street corners. Tim　　5　 to a neighborhood watch group in Stoneville, Indiana, USA. The neighborhood watch group　 6 　 on the third Wednesday of every month. That's　　7 　 Tim get together with about ten of his neighbors to discuss community　 8 　 . Members of the neighborhood watch group want to help the police 　 9 　 their homes, streets, and families safe. Tim Stedman, president of　 10 　 neighborhood watch group, agrees with Tim. "People seem to think that crime(犯罪) happens to other people but not　 11 　 them. Well, it's never happened to me," he said, " but I don't think anyone has the　 12　 to steal from other people or to make them feel　 13　 sitting in their own homes." Alex, a member of the group, said that all the neighbors　 14 　 out for one another. "We　 15　 each other's homes. We keep watch on the neighborhood at night and on weekends. Usually a　　16　 of four or five of us goes out together. If something doesn't look right, then we call the　　17 　 . For example, if we notice a group of teenagers who seem to be looking for　　18 　 , or someone destroying property, we report to the police." Alex feels the neighborhood watch group　　19　 a lot in keeping crime down. Her husband Jim agrees, "Police are good people, but they can't do　 20 　 ." 1. A. yet B. still C. just D. rather 2. A. carefully B. clearly C. nervously D. coldly 3. A. familiar B. unusual C. expensive D. interesting 4. A. new B. open C. old D. broken 5. A. attends B. belongs C. goes D. turns 6. A. meets B. quarrels C. sings D. searches 7. A. where B. why C. when D. how 8. A. politics B. wealth C. health D. safety 9. A. keep B. hold C. let D. protect 10. A. its B. his C. their D. your 11. A. round B. on C. about D. to 12. A. right B. chance C. courage D. mind 13. A. unlucky B. unsafe C. disappointed D. discouraged 14. A. set B. let C. hold D. look 15. A. care B. enter C. watch D. manage 16. A. group B. set C. number D. crowd 17. A. judges B. police C. firemen D. doctors 18. A. work B. burden C. service D. trouble 19. A. produces B. finds C. gets D. helps 20. A. anything B. everything C. harm D. wrong