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")