在建立表间一对多的永久联系时，主表的索引类型必须是（）。

A．SELECT s.学号，姓名，平均成绩 FROM student s，score sc<br> WHERE s.学号=sc.学号<br> 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 student s，score sc<br> WHERE s.学号=sc.学号AND COUNT(*)＞=5<br> GROUP BY 学号 ORDERBY 3 DESC
C．SELECT s.学号，姓名，AVG (成绩) 平均成绩 FROM student s，score sc<br> WHERE s.学号=sc.学号 AND COUNT(*)＞=5<br> 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 student s，score sc<br> WHERE s.学号=sc.学号<br> GROUP BY s.学号 HAVING COUNT(*)＞=5 ORDER BY 3 DESC