第(24)至(26)题基于“学生-选课-课程”数据库中的如下三个关系： 学生基本信息：S(S#, SNAME,SEX,AGE) 选课信息：SC(S#, C#,GRADE) 课程信息：C(C#, CNAME, TEACHER)

若要求“查询选修了C2号课程的学生的学生号”，正确的关系代数表达式是 Ⅰ．π[S#](σ[C#=’C2’](SC)) Ⅱ．σ[C#=’C2’](π[S#](S×SC)) Ⅲ．σ[C#=’C2’](π[S#](SC))

A．SELECT S# FROM SC GROUP BY S# WHERE COUNT(*)＞3
B．SELECT S# FROM SC GROUP BY S# HAVING COUNT(*)＞3
C．SELECT S# FROM SC ORDER BY S# WHERE COUNT(*)＞3
D．SELECT S# FROM SC ORDER BY S# HAVING COUNT(*)＞3

A．S和SC
B．SC和C
C．S和C
D．S、SC和C