下列做法中有利于保护金属资源的是（　　）

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【提出问题】这瓶NaOH溶液一定变质了，其变质程度如何呢？

    There are many different ways to travel to Paris from London. One of the quickest and most popular ways is by train. From Waterloo Railway station, you can travel all the way to Paris. The journey takes about three hours. The train, called the Eurostar, is very modern, and the seats are comfortable.
    You can also travel to France by ship. First, you go to Dover, on the coast (海岸), by bus from London. The bus stops at the Ferry Terminal Seven in Dover. You can buy your tickets there. People who travel by car can drive on to the ferry (渡轮), too.
Another way to travel to Paris is by plane. There are several airports with international flights to Paris and other cities in France. You can travel to London Airport by bus or underground. Some people even go by taxi but this is a very expensive way to travel!

初三年级某班有54名学生，所在教室有6行9列座位，用$\left(m,n\right)$表示第m行第n列的座位，新学期准备调整座位，设某个学生原来的座位为$\left(m,n\right)$，如果调整后的座位为$\left(i,j\right)$，则称该生作了平移$a,b=\left(m-i\right),\left(n-j\right)$,并称$a+b$为该生的位置数。若某生的位置数为10，则当$m+n$取最小值时，$\mathrm{m\times }n$的最大值是多少？