They are Tom's _____. [ ]
A. sister
B. brother
C. parent
D. parents
答案：D
证明：cos3α=4cos3α-3cosα．
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
