为自己加油

给定学生和成绩的数据库表,写一个 sql 语句求出每门课程的平均分( ) 。

A.SELECT SUM  ( score ) FROM course GROUP BY student_id;
B.SELECT score FROM course GROUP BY course_id;
C.SELECT AVG  ( score ) FROM course GROUP BY course_id;
D.SELECT AVG  ( score ) FROM course ;