未知题型 给定关系R（A,B,C,D）和关系S（C,D,E），对其进行自然连接运算R??s后的属性列为（）个;与σR.B＞S.E（R??S）等价的关系代数表达式为（）。A.4B.5C.6D.7@@@SXB@@@A.σ2＞7（R×S）B.π1.2.3.4.7（σ’2‘＞’7‘^3=5^4=6（R×S））C.σ’2‘＞‘7’（R×S）D.π1.2.3.4.7（σ2＞7^3=5^4=6（R×S））

单项选择题 A．Showing children's living conditions, social status, cultural customs.B．Providing a space for children's communication.C．Showing documents, videos, pictures, costumes, toys or handicrafts concerning Chinese children.D．Being interactive and provide a space for children's study and creation.

单项选择题 听力原文：M: Congratulations on breaking the records at the recent Olympics, Miss Green!W: Thank you, Mr Parker.M: Oh, look at those trophies! You became a swimming star at the age of fifteen, didn't you?W: Yeah, you're right.M: Could you tell me something about your plans for the future?W: To tell you the truth, I'm going to give up swimming from now on.M: I beg your pardon?W: I mean I'd rather not swim at all. I'm too old to...M: Excuse me, if I'm not wrong, you're only twenty now. Is it too old twenty?W: Yes, too old for a swimmer. If I swim in an international competition, I’ll hardly be able to win.M: But... er... don't you enjoy swimming?W: Certainly, and I enjoy visiting other countries, and the Olympics are very exciting. However, I missed more important things. I had to work very hard at swimming. There was training before school, after school, and at weekends. While other girls were growing up, I was swimming, swimming. But life isn't merely swimming, is it?When did Miss Green become a swimming star?A．At the age of 15.B．At the age of 20.C．At the age of 18.D．At the age of