字符A、B、C依次进入一个栈，按出栈的先后顺序组成不同的字符串，至多可以组成(28)个不同的字符串。

(75)

(76)

(74)

(73)

Structured programming practices(71)rise to Pascal，in which constructs were introduced to make programs more readable and better(72). C provided a combination of assembly language and high-level Structure to create a general-purpose language that could be used from system to(73)programming. Next came object orientation，which is(74)of a methodology and design philosophy than a language issue．This is(75)by the addition of so-called OO extensions to current languages，such as C．

(71)

TCP/IP是一个协议族，它的体系结构分4层：应用层、网际层、网络接口层和(46)，其中ARP协议属于(47)。为了在源主机和目的主机之间传送数据，IP协议需要确定源主机和目的主机是否在同一个网络中。

题的关系代数操作的基本关系R和S如下：

若关系R和S的关系代数的操作结果如下，这是执行了(20)。

若关系R和S的关系代数操作的结果如下，这是执行了(21)。

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