假定有三个关系，学生关系S、课程关系C和学生选课关系SC，它们的结构如下： S(S#，SN，Sex，Age，D...
A．SELECT S#FROM SC
WHERE C#="C2" AND Grade IN
(SELECT Grade FROM SC WHERE C#="C2")
B．SELECT S# FROM SC
WHERE C#="C2" AND Grade＞=
(SELECT Grade FROM SC WHERE C#="C2")
C．SELECT S# FROM SC
WHERE C#="C2" AND Grade＞=ALL
(SELECT Grade FROM SC WHERE C#="C2")
D．SELECT S# FROM SC
WHERE C#="C2" AND Gtade NOT IN
(SELECT Grade FROM SC WHERE C#="C2")
