在解决计算机主机与打印机之间速度不匹配问题时，通常设置一个打印数据缓冲区，主机将要输出的数据依次写入该缓冲区，打印机则从该缓冲区中取出数据打印，该缓冲区应该是一个______结构。

A(n) ______ is a destructive program that fills a computer system with self-replicating information，clogging the system so that its operations are slowed or stopped．

Electronic commerce may be defined as the entire set of processes that support ______ activities on anetwork and involve information analysis．

______are Intemet addresses used by Web browsers to locate Intemet resources．

A GIS stores information about the world as a collection of thematic ______ that can be linkedtogether by geography．

(66)

The most widely used LAN system is ______ which adopts a form. of access known as CSMA-CD．

(64)

学生成绩的关系模式为s(SNO,SNA,CN,SM)，其中SNO表示学生的学号、SNA表示学生姓名、CN为学生所选课程号、SM为学生所选课程的成绩。从中选取选择了课程号为10081025课程的学生的学号、姓名和成绩，并按成绩由高到低进行排序。

 SELECT SNO,SNA,SM

 FROM S

 WHERE (60) 

  (61) SM DESC;

(65)

在一个图中，所有顶点的度之和等于所有边数的(41)倍；在一个有向图中，所有顶点的入度之和等于所有顶点出度之和的(42)倍。

(63)

(62)