课程表结构如下SC(sno, cno, cname, grade),其中列cno和sno的数据类型均为varchar(20),cname为nvarchar(20),grade数据类型为smallint,查询平均分大于80分且所有课程均及格的学生学号,对应的SQL语句可以是( )。 A. select sno from SC group by sno having AVG(grade)>80 and MIN(grade)>=60; B. select sno from SC group by sno having AVG(grade)>80 and avg(grade)>=60; C. select sno from SC group by sno having avg(grade)>80 and min(grade)>=60; D. select sno from SC group by sno having min(grade)>80 and avg(grade)>=60;