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问题 单项选择题

有3个关系R、S和T如下：

其中关系T由关系R和S通过某种操作得到，该操作为

A．选择

B．投影

C．交

D．并

答案

参考答案：D

解析： 两个相同结构关系的并是由属于这两个关系的元组组成的集合。

单项选择题 A1/A2型题

关于发射式计算机断层的叙述，错误的是（）

A.ECT获得脏器三维图像

B.ECT不能进行断层显像

C.是以引入体内的放射性药物为放射源

D.基本原理与γ照相机相似

E.从体外不同角度采集脏器中放射性药物分布信息
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单项选择题

Traditional structured analysis techniques focus upon the flow of (71) within a system. Object - oriented analysis emphasizes the building of real - world models. It examines requirements from the perspective of the classes and objects found in the vocabulary of the (72) domain.

Traditional system design method emphasizes the proper and effective structure of a complex system. Object -oriented design method encompasses the process of object -oriented decomposition and a (73) for depicting both logical and physical as well as static and dynamic models of the system under design.

Object - oriented programming is a method of implementation in which programs are organized as cooperative collections of objects, each of which represents an (74) of some class, and whose classes are all members of a hierarchy of classes united via (75) relationships.

73（）

A．mark

B．picture

C．symbol

D．notation
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