以下MySQL语句中,已知关系模式stu_course(Sno,Cno,Term,Grade),查询统计总成绩和平均成绩的记录,正确的是( )。 A. SELECT SUM(grade) AS ‘总成绩’,AVG(grade) AS ‘成绩平均值’ FROM stu_course; B. SELECT grade AS “总成绩” FROM stu_course; C. SELECT grade AS “成绩平均值” FROM stu_course; D. SELECT SUM(grade) AS 总成绩,AVG(grade) AS 成绩平均值 FROM stu_course;