第19题: [单项选择]在关系模型中,实现“关系中不允许出现相同的元组”的约束是通过______。 A. 候选键 B. 主键 C. 外键 D. 超键 参考答案:B 答案解析:[解析] 关系中的一个属性集的值能唯一标识关系中的元组且又不含多余属性值,我们称该属性集为关系的关键字或候选关键字。在Visual Fox-Pro中,通常是利用主关键字或候选关键字来保证表中记录的唯一
第21题: [单项选择]______ was to return to school. A. That really interested him B. What really interested him C. Which really interested him D. That interested him really 参考答案:B 答案解析:[解析] 本题考查what引导的主语从句。主语从句一般由that来引导,但是that后的从句在语法上需是一个完整的句子。如:That he was responsible for the accide