题目

England was one of the first countries to set aside a day to recognize mothers．In the eighteenth century,  21   many people worked as household servants for the   22 , Mothering Sunday was _ 23   for them to return home to be with their mothers．Though this   24  stopped when the Industrial Revolution altered the working and living patterns of people, one  25  for mothers was established as a holiday in the twentieth century． In the United States, Mother's Day did not become a（n）   26   holiday until 1915．Its establishment was   27  due to the perseveran某商场销售某种品牌的纯牛奶，已知进价为每箱40元，生产厂家要求每箱售价在40元至70元之间．市场调查发现：若每箱以50元销售，平均每天可销售90箱，价格每降低1元，平均每天多销售3箱，价格每升高l元，平均每天少销售3箱．（1）写出平均每天销售量y（箱）与每箱售价x（元）之间的函数关系式．（注明范围） （2）求出商场平均每天销售这种牛奶的利润W（元），与每箱牛奶的售价x（元）之间的二次函数关系式．（每箱的利润=售价-进价）（3）求出（2）中二次函数图象的顶点坐标，并求当x=40，70时W的值．在给出的坐标系中画出函数图象的草图．（4）由函数图象可以看出，当牛奶售价为多少时，平均每天的利润最大？最大利润为多少？