根据SQL标准，要查询表student中平均年龄age小于21的所在系dept及其平均年龄值，下面哪条语句适

根据SQL标准，要查询表student中平均年龄age小于21的所在系dept及其平均年龄值，下面哪条语句适用？()

A select dept,avg(age) from student where avg(age)<21

B select dept,avg(age) from student group by dept having avg(age)<21

C select dept,avg(age) from student having avg(age)<21

D select dept,avg(age) from student group by dept where avg(age)<21