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【单选题】
关系模式R(A1,A2,A3,A4,A5,A6),如果A2→(A3,A5); (A1,A3)→A6;A.
2，A6)→A4，则R的候选键为_________。
B.
A2
C.
(A1，A2)
D.
(A1，A3)
E.
(A2，A6)
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