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