7、若有学生关系s(id,name,age,dept_name),其中,id为学号,name为姓名,age为年龄,dept_name为所属学院。则,下列查询每个学院中年龄在20~25之间的学生平均年龄的sql语句正确的是 a、select dept_name,avg(age) from s where age >20 and age<25 b、select dept_name,avg(age) from s where age >20 and age<25 group by dept_name c、select dept_name,avg(age) from s group by dept_name having age >20 and age<25 d、select dept_name,avg(age) from s having age >20 and age<25