下列是火柴头中含有的四种物质，火柴被点燃时，能闻到一股气体的刺激性气味，已知这种气体会污染空气，形成酸雨，火柴点燃时生成这种有害的物质是（ ）
A．二氧化锰
B．氯酸钾
C．红磷
D．硫
A、二氧化锰不具有可燃性，不能燃烧，故选项错误．
B、氯酸钾不具有可燃性，不能燃烧，故选项错误．
C、红磷在空气中燃烧，产生大量的白烟，生成一种白色固体，故选项错误．
D、硫在空气中燃烧，产生淡蓝色火焰，产生一种具有刺激性气味的气体，该气体会污染空气，形成酸雨；故选项正确．
故选D．
| 完形填空。 | |||
| How can you think in English? I think the best way is to practice as what a football player does every day. During the 1 the football player will pass the ball to his teammates over and over again. So he won't have to 2 passing the ball in the game, he will just do it. You can 3 yourself to think in English this way. The first step is to think of the words that you use daily, simple everyday words 4 book or shoe or tree. For example, whenever you 5 a "book" you should think of it in English instead of in your mother language. After you have learned to think of several words in English, then move on to the next step-thinking in 6 . Listening and repeating is a very useful 7 to learn a language. Listen first and don't care too much about 8 you fully understand what you're hearing. Try to repeat what you hear. The more you listen, the 9 you learn. After you reach a higher level, 10 having conversations with yourself in English. This will lead you to think in English. | |||
| ( )1. A. practice ( )2. A. wait for ( )3. A. allow ( )4. A. in ( )5. A. buy ( )6. A. sentences ( )7. A. idea ( )8. A. which ( )9. A. harder ( )10. A. remember | B. break B. think about B. train B. about B. keep B. passages B. step B. whether B. less B. stop | C. game C. look at C. tell C. like C. borrow C. lessons C. way C. how C. later C. start | D. day D. give up D. ask D. from D. see D. classes D. pan D. why D. more D. finish |