单项选择题

Let R and S be the relation respectively: Relation R: A B C Relation S: B C D 1 2 3 2 3 4 6 7 8 2 3 5 The number of tuplesthat the result of the expression R ∞ S (Here is the natural join of relations R and S) is

A．3
B．4
C．6
D．9

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单项选择题

所谓I O操作是指内存与I O设备之间进行信息交换。在I O方式的发展过程中，对CPU依赖程度最高的I O操作是

A．通道控制
B．中断控制
C．程序控制
D． DMA控制

单项选择题

Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n

A． topology
B． protocol
C． service
D． peer

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