在以下语句中，哪个是降序查询t1表中不同城市pm10的平均值（）
A.        select city_name,avg(pm10) from t1
group by city_name
order by avg(pm10) desc;
B.        select city_name,mean(pm10) from t1
group by city_name
order by avg(pm10) desc;
C.        select city_name,avg(pm10) from t1
group by city_name
order by avg(pm10) asc;
D.        select city_name,avg(pm10) from t1
group by city_name
order by avg(pm10);

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