单项选择题

Suppose we have the relation schema R(A,B,C,D) with functional dependencies;F{A→B,B→C,C→D,D→A},then R(A,B,C,D)at least is in

A．1NF
B．2NF
C．3NF
D．BCNF

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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

单项选择题

A disk scheduling mechanism in an operatingsystem causes the disk arm to sweepback and forth across the disk surface servicing all requests in its path. This is a

A．First Come First Serve
B．Shortest Seek Time First
C．Scan
D．None of the above

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