下列函数中，y一定是关于x的二次函数的是（　　）

阅读文章《济南的冬天》（选段），完成习题。
     ⑴古老的济南，城里那么狭窄，城外又那么宽敞，山坡上卧着些小村庄，小村庄的房顶上卧着点雪，对，这是张小水墨画，也许是唐代的名手画的吧。
     ⑵那水呢，不但不结冰，倒反在绿萍上冒着点热气，水藻真绿，把终年贮蓄的绿色全拿出来了。A天儿越晴，水藻越绿，就凭这些绿的精神，水也不忍得冻上，况且那些长枝的垂柳还要在水里照个影儿呢！B看吧，由澄清的河水慢慢往上看吧，空中，半空中，天上，自上而下全是那么清亮，那么蓝汪汪的，整个的是块空灵的蓝水晶。这块水晶里，包着红屋顶，黄草山，像地毯上的小团花的小灰色树影；这就是冬天的济南。

    About 70，000，000 Americans are trying to lose weight．This is almost one out of every three people in the United States．Some people eat 1food and they hardly have any fats or sweets. Others do running exercise with machines，take medicines，or even have operations2you can see losing weight is 3 work，and it will also cost a lot of money．But 4do so many people in the United States want to lose weight?
    Many people in the United States worry about their look of the body．For many people，looking nice also means to be 5．Other people worry about their health as many doctors 6 overweight is not good．
    Most people want to find a faster and 7 way to take off fat，and books of this kind are very popular．These books tell people how to lose weight．Each year a lot of new books like these are 8．Each one says it can easily help people take fat away．
    Losing weight can be expensive．Some overweight people go to health centers，like La Costa in California．Men and women9several hundred dollars a day at these health centers．People live there for one week or two，taking exercise，eating different foods．Meals there may be just a little．All this works for losing weight．104 days on the program，one woman called Mrs. Warren lost 5 pounds(2.27kg)．At 400 dollars a day, she spent 320dollars to lose each pound．But she said she was still glad to do so．
    Health centers，books，medicines，operations，running and exercise machines all get a lot of money．So in the United States，losing weight may mean losing money too.