需求
参考答案：
一种商品的需求函数是指消费者在一定时期内各种可能的价格水平愿意而且能够购买的该商品的数量。
He had promised to come and help us, and ___.
A．so did he
B．so he did
C．so had he
D．so he had
直埋蒸汽管道阀门的选择及安装有何要求？
