阅读理解。

      I am an English girl. My name is Mary. I'm twelve. My dad and mom are English teachers.

I have a brother(哥哥). His name is Tom and he is thirteen years old. We are in China now.

We are in Beijing No. 4 Junior High School. Tom is in Class 4, Grade 7 and I am in Class 9,

Grade 7. I have a dog. My dog looks strong. It has a big mouth and two big black ears. We

have many(许多) Chinese friends here. They are good.

根据短文内容,判断正(T)误(F).

( )1. Mary comes from the U.S.A.

(     )2. Mary and Tom are in the same class, but they are in different grades.

(     )3. Mary's dog's mouth and ears are big.

(     )4. The dog's ears are white.

(     )5. Tom and Mary have many friends in China.

在SQL中，可以用谓词UNIQUE来测试一个集合中是否_________。

A．为空集合

B．存在重复分量值

C．为非空集合

D．存在重复元组