如下两个关系R1和R2，它们进行何种运算后得到R3R1
参考答案：D
解析： 连接是从两个关系R和S的笛卡儿积中选取属性值间满足一定条件的元组。
阅读短文，判断正(T)误(F)。
Dear friends,
I am Nancy. I am from America. I can speak Chinese. I like travelling best. I want to go to Australia
by plane. I send e-mails to my friends in Australia once a week.
( ) 1. Nancy is from America.
( ) 2. She can speak Chinese.
( ) 3. She wants to go to Australia by ship.
( ) 4. She sends e-mails to her friends twice a week.
（How time flies! We’ll graduate. Three years __________ really a short time.
A. was
B. are
C. is
D in
