查询book表中每类图书的平均单价，结果只显示平均单价大于30的图书类别名和平均单价，以下SQL语句正确的是？ A: select 图书类别,avg(单价)from bookwhere 单价 >= avg(单价); B: select 图书类别,avg(单价)from bookgroup by 图书类别; C: select 图书类别,avg(单价)from bookgroup by 图书类别having avg(单价) > 30; D: select 图书类别,avg(单价)from bookgroup by 图书类别where avg(单价) > 30;