关系R、S如下图所示，关系代数表达式 （ ），对关系R、S 进行自然连接后的属性列数和元组个数分别为（ ）。
问题1
A.
B.
C.
D.
问题2
A.3和0
B.3和2
C.6和0
D.6和2
正确答案及解析
正确答案
解析
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