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