问题 单项选择题

当关系模式R(A,B)已属于3NF,下列说法中哪一个是正确的

A.它一定消除了插入和删除异常

B.仍存在一定的插入和删除异常

C.一定属于BCNF

D.A)和C)都是

答案

参考答案:B

解析:若关系模型R∈2NF,且每一个非主属性都不传递于码,则R∈ 3NF。3NF就是不允许关系模式的属性之间有这样的非平凡的函数依赖X→Y,其中X不包括码,Y是非主属性。X不包含有两种情况,一种情况X是码的真子集,这是2NF不允许,另一种情况X不是码的真子集,这是3NF不允许的。在3NF中,仍存在一定的插入和删除异常。

选择题
阅读理解

阅读理解。

    Coffee has been a part of people's lives for thousands of years, and today it is still one of the favorite drinks

in the world. Many people in the world begin their busy day by drinking a cup of coffee or several cups of

coffee. They have always said that a cup of coffee in the morning helps them begin their day in the right way.

So they say a cup of coffee is necessary for them in the morning.

    Do you know what in the coffee makes our bodies and brains active? It is caffeine. Caffeine may increase a

person's mental and physical abilities (精力和体能). For example, two cups of coffee will make you breathe

faster and increase body heat. All this makes your heart beat faster, and doctors warn that this may be a little

dangerous.

    Caffeine is found in tea, coffee, chocolate and some other foods. A little caffeine is probably not harmful.

But too much caffeine can make people nervous and sleepless.

1. A good title for this passage is _____. [ ]

A. Coffee Is Harmful

B. Coffee and Tea

C. Caffeine and Health

D. Caffeine Is Useful

2. From this passage, we may know that drinking a cup of coffee late at night might _____.[ ]

A. give you a headache

B. make you fall asleep

C. keep you awake

D. be dangerous

3. Caffeine is found in _____. [ ]

A. tea and water

B. chocolate, meat and some other foods

C. coffee and jam

D. coffee, tea, and some other foods

4. Which of he following is NOT TRUE? [ ]

A. Coffee has been a part of people's lives for thousands of years.

B. Caffeine does no harm to our body.

C. Two cups of coffee will make you breathe faster and increase body heat.

D. Caffeine may increase humans' mental and physical abilities.