设有关系模式SC(sno，eno，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="cl'，AND SELECT sno FROM sc WHERE cno="c2"
C．SELECT sno FROM sc WHERE cno="cl" UNION SELECT sno FROM sc WHER：E cno="c2"
D．SELECT sno FROM sc WHERE cno="cl"AND sno IN(SELECT sno FROM sc WHERE cno="c2")