Black Friday

    Thanksgiving is always on a Thursday.The day after it has been known as Black Friday. It is the start of the holiday shopping season,and it has been the busiest shoppingday of the year in the US since 2005.

    Most stores offer big sales on Black Friday.They open their doors quite early in the morning.They try to attract shoppers with big discounts(打折).Some items(商品) like TVs and clothes are much cheaper than usual.Stores may even lose money on these items.They hope that shoppers will buy gifts for other people while they are in the store.However,there are problems.The biggest one is that there are not enough low—priced items. Each store may only have a few so that not everyone can get one.

    The situation makes people worried.Some Black Friday events have been crazy. There have been fights over toys among shoppers. Some workers have even been hurt by large crowds. But most Black Friday events are safe and fun..So where does the name “Black Friday”come from? It was firstused in Philadelphia in the 1950s.

    Now people all over the country take part in Black Friday events.It is even spreading to other parts of the world.Stores have held Black Friday events in the UK，Australia ，and Brazil since 2012. In Mexico，stores offer a weekend of discounts every year. They call it “El Buen Fin”,which means“the good weekend ”in Spanish. I guess people everywhere like to buy things on sale.

下列函数中最小正周期是π且图象关于点($\frac{\mathrm{\pi }}{3}$,0)成中心对称的一个函数是（　　）

现有铜与金属镁、铁、锌中的一种组成的混合物，现欲测定其组成。

【实验步骤及数据】取该混合物粉末8.0 g放入烧杯中，将140.0 g 溶质质量分数为14.0％的稀硫酸平均分四次加入该烧杯中，充分反应后，测得剩余固体质量数据记录如图：

通过计算(写出计算过程)，求：