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．SELECTSnoFROMSc WHERECno="c1"ANDCno="c2"
B．SELECTSnoFROMSc WHERECno="c1"ANDSELECTSnoFROMSc WHERECno="c2"
C．SELECTSnoFROMSc WHERECno="c1"UNIONSELECTSnoFROMSc WHERECno="c2"
D．SELECTSnoFROMSc WHERECno="c1"ANDSno IN(SELECTSnoFROMSc WHERECno="c2")