34）查詢订购单的数量和所有订购单平均金额的正确命令是 (A)SELECT COUNT(DISTINCT 订单号),AVG(数量*单价) FROM 产品 JOIN 订购单名细 ON 产品.产品号=订购单名细.产品号 (B)SELECT COUNT(订单号),AVG(数量*单价) FROM 产品 JOIN 订购单名细 ON 产品.产品号=订购单名细.产品号 (C)SELECT COUNT(DISTINCT 订单号),AVG(数量*单价) FROM 产品,订购单名细 ON 产品.产品号=订购单名细.产品号 (D)SELECT COUNT(订单号),AVG(数量*单价) FROM 产品,订购单名细 ON 产品.产品号=订购单名细.产品号