按课程号分类，统计相应的选课人数（）

A.select cid， count（*） from student group by cid

B.select cid， count（distinct sid） from sc group by cid

C.select cid， count（distinct sid） from student group by cid

D.select cid， count（distinct cid） from sc group by cid

正确答案：select cid， count（distinct sid） from sc group by cid