单项选择题

What operation on relations R and S produces the relation shown below ( ){tuple | tuple∈R∧tuple∈S}

A．union
B．difference
C．intersection
D．division
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The main goals of Network are sharing computer resources. The resources include hardware、 data and ( ).
A．gateway
B．router
C．software
D．server
单项选择题
Which values are not permitted to be part of the primary key ( )
A．alpha-numeric characters
B．punctuation
C．special characters
D．NULL
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