对于下图所示的 Students 和 SC 关系，属性 Sno 表示学号，Sname_爱下问搜题

搜题请搜索小程序“爱下问搜题”

问题单项选择题

对于下图所示的 Students 和 SC 关系，属性 Sno 表示学号，Sname 表示姓名，Sex 表示性别，SD 表示专业，Age 表示年龄，Cno 表示课程号，Grade 表示成绩。

a．当用户查询 “选了课程 2 的学生数”时，其结果应为（1）；当 Students 和 SC 关系进行自然连接时，其结果集的属性列数应为（2）。 b．查询 “至少选修了两门课程的每个学生的平均成绩”的SQL 语句如下： SELECTSno，（3） FROM　SC GROUP BY Sno （4）>1；

2（）

A．6

B．7

C．8

D．9

答案

参考答案：B

完形填空

Complete the following passage by using the words in the box. Each word can only be used once. Note that there is one word more than you need.

A. roots B. but C. covered D. reproduce E. energy
F. probably G. gaining H. surrounding I. supplied J. life

Life on land probably began about 430 million years ago, though it has existed in the water for perhaps as much as 3,000 million years. When we think of the first 小题1: on land, we 小题2: think of strange animals coming out of the oceans, but, in fact, no animals could have been living if plants had not been on land first. Plants had to be on land before animals arrived. They 小题3: the first land animals with the 小题4: and food necessary, since they ---the plants are the only form of life that is able to get and store 小题5: .
The first plants to exist out of the water were probably certain kinds of algae （藻类）which were followed by other plants that grew close to the ground and needed water in which to 小题6: . Once the move to land has been made, however, evolution took place quickly. By the end of 100 million years, plants had developed their 小题7: , and some have got tree-like forms since height was very important in 小题8: sunlight. About 300 million years ago, much of the world was 小题9: with forests of huge trees. In most ways they were like modern trees. They had roots, leaves, wood, but mostly they had not developed seeds.

问答题简答题

地震波的速度与介质孔隙度有什么关系？