问题 单项选择题

设有如下三个关系表,下列操作中正确的是【 】。

A.T=R∩S

B.T=R∪S

C.T=RxS

D.T=R/S

答案

参考答案:C

解析:对于两个关系的合并操作可以用笛卡尔积表示。设有n元关系R和m元关系S,它们分别有p和q个元组,则R与S的笛卡儿积记为R×S,它是一个m+n元关系,元组个数是p×q,由题意可得,关系T是由关系R与关系S进行笛卡尔运算得到。

阅读理解

阅读理解。

     This August has seen athletes doing their sweaty part in the "Good Luck Beijing" sports events.

Some foreign athletes have commented on the passion of Chinese spectators (观众).

     Not all the spectators, however, came out looking exactly good. Girls holding umbrellas on beach

volleyball stands. Spectators crying sharply during the hockey matches. These can be unintended disturb.

To avoid them, we'd keep in mind some special manners:

     Beach volleyball

     1. Dress down for beach volleyball. It's also a good time to show off your colorful top and sexy

shorts.

     2. Use protective cream for your skin. Wear sunglasses and bring something to drink. But don't carry

something that blocks others' view.

     3. Look all you want at the body shapes around you, but save some attention for the players' skills.

     Archery & shooting

     1. Sit at the back or on either side of the field. For safety's sake, do not risk out of there.

     2. Above all, keep quiet during the match. Put your cellphone on vibrate or turn it off.

     3. Do not use the flash on your camera, because it can disturb the athletes.

     4. When the athletes are aiming at the target, keep your voice down.

     Baseball, basketball & football

     1. Drinks in glass bottles or cans are not allowed.

     2. Don't bring a whistle. Referees use them and you can puzzle the players.

     3. Players can be tense when taking a shot. Keep the cheering down at critical moments. Do not

use the camera flash.

     4. If you easily get nervous, bring some snacks. Keep your mouth moving to calm you down.

     Cycling & marathon

     1. Spectators should keep off the track. Control your pets.

     2. Cyclists or runners might look tired, sweaty and thirsty, but, do not hand them a bottle of water,

a cool towel or anything else. Any help from spectators can disqualify (剥夺资格)an athlete.

     3. Again, if a cyclist or runner falls, do not rush over to help. The best thing to do is to contact

medical personnel.

1. When watching beach volleyball matches, you should not ______.

A. bring anything to drink

B. use an umbrella on stands

C. wear sunglasses

D. look at the body shapes

2. You'd better not ______.

A. eat anything when watching matches

B. use camera flash when watching cycling

C. shout loudly when watching archery

D. stay where you sit when watching shooting

3. If you go to watch marathon, you can not _______.

A. bring your pets

B. give athletes something to drink

C. cry sharply

D. ask medical personnel to help athletes

4. The passage mainly tells us ______.

A. how spectators disturb athletes

B. how spectators help athletes

C. dos and don'ts while watching matches

D. some rules for sports events

单项选择题