一个查询语句执行后显示的结果为:
1班 80
2班 75
3班 NULL
则最有可能的查询语句是()
A.  SELECT AVG(成绩) FROM 成绩表 WHERE class<=3
B.  SELECT AVG(成绩) FROM 成绩表 WHERE class<=3 GROUP BY class
C.  SELECT AVG(成绩) FROM 成绩表 WHERE class<=3 order by class
D.  SELECT AVG(成绩) FROM 成绩表 HAVING class <=3 GROUP BY class

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