问题 选择题

下 * * 个命题,其中正确的个数为(  )

①a∈A⇒a∈A∪B    ②A⊆B⇒A∪B=B    ③a∈B⇒a∈A∩B        ④A∪B=B⇒A∩B=A         ⑤A∪B=B∪C⇒A=C.

A.2

B.3

C.4

D.5

答案

因为A⊆A∪B,所以a∈A⇒a∈A∪B,故命题①正确;

若A⊆B,则集合A的元素都在集合B中,所以A∪B=B,故命题②正确;

元素a在集合B中,不见得在集合A中,所以也不见得在集合A∩B中,所以a∈B⇒a∈A∩B 不正确;

A∪B=B,说明集合A是集合B的子集,所以有A∩B=A,故命题④正确;

A∪B=B∪C,若A、C都是B的子集,集合A和集合B不一定相等,所以命题⑤不正确.

所以正确的命题个数为3个.

故选B.

阅读理解

阅读短文,选择正确答案。

      What would you do if a stranger gave you some medicine and said it would keep you slim or help you do

well in exams? Would you take it?

      Students at Shenzhen Luohu Foreign Language Middle School have learned to keep themselves safe by

saying "no". In fact, what those strangers want to give them are drugs (毒品). Drugs are dangerous to your

health! 

      "Because teens don't know much about drugs, they might want to try them." said Zhang Dongsheng, the

director of Shenzhen Drug Control Office. 

      Now, many schools arrange (安排) courses or organize some activities to help students know about drugs. 

      Liang, 16, was a good student before trying drugs. One day, some old friends told him it would be fun.

After taking drugs only once, he found it very hard to stop. Liang began to spend all his time and money on

drugs. Soon, he stopped going to school and started stealing (偷窃). In the end, he was put in prison. 

      In China, taking drugs is becoming a big problem for teens. According to a survey, about one million people

in China were using illegal (违法) drugs last year, and 72% of them were young people. Worse still, there are

about 10,000 drug users under 16 now.

      Zhang has some good advice on staying clean. He said, "Drugs are dangerous. Never try them, even if

you're curious! It is the most important for you to keep safe and healthy."

1. Which statement is right about drugs? [ ]

A. Drugs do harm to your health.

B. Drugs help you do well in exams.

C. Drugs keep you slim and lovely.

D. Drugs make you stay well.

2. According to this passage, many schools arrange courses or some activities to ______ now. [ ]

A. know more about cigarettes

B. relax

C. know about drugs

D. learn some Chinese culture

3. Teens want to try drugs because _______. [ ]

A. they are 18 years old

B. they are curious about drugs

C. they have enough money

D. they know about the harm of drugs

4. From Liang's story, we can know that it is not __________ to give up drugs even if one tries only once. [ ]

A. easy

B. hard

C. foolish

D. possible

5. What does the phrase staying clean here mean? [ ]

A. Being clean and tidy.

B. Keeping clothes clean.

C. Wearing clean clothes.

D. Staying away from drugs.

填空题