学生成绩管理数据库系统有下面有三个关系：
选项：

A:select sname,cname,score from student a ,course b ,scores c where a.sno=c.sno and b.cno=c.cno
B:select sname,cname,score from student ,course ,scores
C:select sname,cname,score from student a join course b on a.sno=c.sno join on scores c b.cno=c.cno
D:select sname,cname,score from student a join scores b on a.sno=b.sno join on course c b.cno=c.cno