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设有关系模式SC(sno,cno,grade),查询既选修了c1又选修了c2课程的学生学号,正确的SQL命令是

A. SELECT sno FROM sc WHERE cno="c1"AND cno="c2"
B. SELECT sno FROM sc WHERE cno="c1"AND SELECT sno FROM sc WHERE cno="c2"
C. SELECT sno FROM sc WHERE cno="c1"UNION SELECT sno FROM sc WHERE cno="c2"
D. SELECT sno FROM sc WHERE cno="c1"AND sno IN(SELECT sno FROM sc WHERE cno="c2")

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