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找出每个学生超过他选修课程平均成绩的课程号,以下哪个语句是正确的( )
A.
SELECT sno, cno from SC x where Grade>=(select avg(grade) from SC y where y.sno=x.sno)
B.
SELECT sno, cno from SC where Grade>=(select avg(grade) from SC group by sno)
C.
select sno,cno from SC group by sno having Grade>=AVG(Grade)