求每个交易所的平均单价的SQL语句是( )

题目：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