求每个交易所的平均单价的SQL语句是______。

A) SELECT 交易所,avg(单价)FROM stock GROUP BY 单价
B) SELECT 交易所,avg(单价)FROM stock ORDER BY 单价
C) SELECT 交易所,avg(单价)FROM stock ORDER BY 交易所
D) SELECT 交易所,avg(单价)FROM stock GROUP BY 交易所