对于课本48页的三张表,要查询每门课程的课程名和平均成绩,下面能完成此查询的SQL语句是( ) A. select cname,sum(grade) from C,SC group by cname; B. select cname,avg(grade) from C,SC where C.cno=SC.cno group by cname; C. select cname,avg(grade) from C,SC group by cname having C.cno=SC.cno; D. select cname,avg(sno) from C,SC where C.cno=SC.cno order by cname;