在表单中，______属性是用来确定复选框是否被选中的。

A. SELECT S.学号，姓名，平均成绩 FROM S，SC
WHERE S.学号=SC.学号
GROUP BY S.学号 HAVING COUNT(*)＞5 ORDER BY 平均成绩 DESC
B. SELECT 学号，姓名，AVG (成绩) FROM S，SC
WHERE S.学号=SC.学号 AND COUNT (*)＞5
GROUP BY 学号 ORDER BY 3 DESC
C. SELECT S.学号，姓名，AVG (成绩) 平均成绩 FROM S，SC
WHERE S.学号=SC.学号AND COUNT(*)＞5
GROUP BY S.学号 ORDER BY 平均成绩 DESC
D. SELECT S.学号，姓名，AVG(成绩)平均成绩 FROM S，SC
WHERE S.学号=SC.学号
GROUP BY S.学号 HAVING COUNT(*)＞5 ORDER BY 3 DESC