统计选修了a001课程的学生的平均成绩(  ) A. select cid,avg(grade) from student where cid=’a001’ group by cid B. select cid,avg(grade) from student group by cid having cid=’a001’ C. select cid,avg(grade) from sc where cid=’a001’ group by cid D. select cid,avg(grade) from course where cid=’a001’ group by cid