10.有自考成绩单:R(学号,课程编号,考试日期,成绩),规定一名考生可以选考多门课,且一门课有多个考生选考,允许没有考试通过者再次参加考试,则R的主键应该是
A.学号
B.课程编号
C.学号,课程编号
D.学号,课程编号,考试日期
正确答案是D
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