单项选择题
在学生选课系统中,有如下关系模式:student学生表(,sName,sSex,sAge,sDept)course课程表(,cName,cPno,cCredit)sc选课表(,,,scGrade)其中,sNo是学号、sName是学生姓名、sSex是性别、sAge是年龄、sDept是学生所在院系、cNo是课程编号、cName是课程名称、cPno是先修课程、cCredit是学分、scNo是选课代码、scGrade是成绩。查询选课表中成绩在前五名的学生学号,正确的SQL语句是( )。 A. select top 5 sNo from sc order by scGrade desc B. select sNo from sc order by scGrade desc limit 5 C. select sNo from sc order by scGrade limit 5 D. select sNo from sc order by scGrade asc limit 5