第28题: [单项选择]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 参考答案:B