问题 单项选择题

两个关系有若干相同属性名等值连接,并去掉重复属性的关系代数运算称为 ______。

A.取列运算

B.投影运算

C.自然连接

D.选择运算

答案

参考答案:C

阅读理解

阅读理解。

     Every year thousands of people get hurt or die when they are crossing the road. Most of these people are

old people and children. Old people often get hurt or die because they can't see or hear very well. Children

usually meet with accidents because of their carelessness. They forget to look and listen before they cross the

road.

     How can we lessen (减少) traffic accidents? All of us must obey the traffic rules. For the drivers, they

shouldn't drive too fast. If they drive too fast, it will be very difficult to stop the cars in a very short time. For

the pedestrians (行人), it's very important to be careful when they are walking on the road. Therefore, when

we walk across the road, we must try to walk along the pavements. We must stop and look both ways before

crossing the road. Look left first, next look right, and then look left again. Only when we are sure that the road

is clear can we cross it. The right way to cross the road is to walk quickly. It's not safe to run. If people run

across the road, they may fall down. Teens should try to help children, old people or blind people to cross the

road, and never play in the street.

1. Which of the following is NOT the right way to cross the road?

A. Stop and look both ways.

B. Run quickly.

C. Make sure that the road is clear.

D. Walk quickly.

2. Why do most old people often get hurt or die when they are crossing the road?

A. Because they are not careful.

B. Because they forget to look and listen.

C. Because they can't see or hear very well.

D. Because they often run across the road.

3. As a student, you can _____ to try to lessen traffic accidents.

A. help children to cross the road

B. play in the street carefully

C. run across the road quickly

D. let drivers drive slowly

选择题