单项选择题

A.r={R1(A1,A2,A3),R2(A3,A4,A5),R3(A5,A6,A7)}
B.r={R1(A1,A2,A3,A4,A5),R2(A5,A6,A7)}
C.r={R1(A1,A2,A3),R2(A3,A4,A5,A6,A7)}
D.r={R1(A2,A3,A6,A7),R2(A1,A4,A5)}
E.其他都不对。