查询所有成绩均大于80分的学生姓名,应执行( )语句 A: SELECT SNAME FROM STUDENT WHERE EXISTS (SELECT *FROM SC WHERE STUDENSNO = SC.SNO AND GRADE> 80); B: SELECT SNAME FROM STUDENT WHERE EXISTS (SELECT *FROM SC WHERE STUDENSNO = SC.SNO AND GRADE C: SELECT SNAME FROM STUDENT WHERE NOT EXISTS (SELECT *FROM SC WHERE STUDENSNO = SC.SNO AND GRADE D: SELECT SNAME FROM STUDENT WHERE NOT EXISTS (SELECT *FROM SC WHERE STUDENSNO = SC.SNO AND GRADE> 80);