第15题: [单项选择]在关系运算中,查找满足一定条件的元组,相关的运算称为( )。 A. 选择 B. 投影 C. 连接 D. 扫描 参考答案:A 答案解析:[解析] 专门的关系运算包括选择、投影和连接。选择是指从关系中找出满足条件的元组;投影是从关系中指定若干个属性组成新的关系;连接是将两个关系模式拼接成一个更宽的关系模式,生成的关系中包含满足连接条件的
第18题: [单项选择]{{B}}Questions 11 to 18 are based on the conversation you have just heard.{{/B}} A. The barbecue has been canceled. B. The weather will probably be cool. C. The man will not be able to attend the barbecue. D. Casual dress will be appropriate. 参考答案:D 答案解析:[听力原文]
M: I’ll be coming straight from work,so I’ll have to pack a change of clothes.
W:It’s only