某网站的数据库有一个成绩表myscore，希望找出成绩表中平均得分小于90的所有试卷（）

A. select paper_id from my score where sum（score）＜90 group by paper_id
B. select paper_id from my score group by paper_id having avg（score）＜90
C. select paper_id from my score where avg（score）＜90
D. select paper_id from my score where avg（score）＜90 group by paper_id