“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#(_{σC# =’C2’}(SC. ) Ⅱ．_{σC#=’C2’}(Π_S#(S×SC. ) Ⅲ．_{σC#=’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