现有表 student. 包含学号(sid)、班级(class)、 课程编号(course)和成绩(score)等字段， 若想要查询每个班级每门课的平均成绩.下列语句能实现的是？
A.select class, course, avg (score) from student group by class, course. B.select class, course. avg(score) from student group by course。 C.select class, course, avg(score) from student group by course order by class; D.select class, course, avg(score) from student group by class;正确答案A
