第9题: [单项选择]What do we learn about deserts from this text A. Deserts can change into green and fertile areas. B. Certain areas have always been desert. C. Deserts were once the home of elephants and giraffes. D. Deserts have been growing since the beginning of the worl 参考答案:A 答案解析:文章第二段第六行“Throughout history deserts have been expanding and retreating again.”这段中谈到,一些肥沃的地方以前曾经是沙漠,而
第35题: [单项选择]设有关系模式以A,B,C) ,根据语义有如下函数依赖集:F=(A→B,(B,C) →A。关系模式R的规范化程度最高达到( )。 A. 1NF B. 2NF C. 3NF D. 4NF 参考答案:C 答案解析:[解析] 依据Armstrong公理系统,可知关系R的主码为(B,C),并且不存在部分函数依赖和传递依赖,所以关系R的规范化程度达到3NF,根据BCNF的定义,要求每一个非平凡的函数依赖X→Y,都有