关系SC(Sno，Cno，Grade)表示学号，课程号，成绩。现要查询平均分在90分以上的学生的学号和平均成绩，正确的语句是______。

A．SELECT Sno，AVG(Grade) FROM SC WHERE AVG(Grade)＞=90 GROUP BY Sno

B．SELECT Sno，AVG(Grade) FROM SC GROUP BY Sno HAVING AVG(Grade)＞=90

C．SELECT Sno，AVG(Grade) FROM SC WHERE AVG(Grade)＞=90 ORDER BY Sno

D．SELECT Sno，AVG(Grade) FROM SC GROUP BY Cno HAVING AVG(Grade)＞=90

什么是导游服务？