有一次小明上街买回来一袋黄豆芽放在阳光下，下午去打开一看，发现许多黄豆芽变成了“绿豆芽”，小明觉得奇怪，他把这一发现告诉了同学，他们决定把这个问题搞清楚，于是他和同学们一起去探究．

    It is said that about half of the people in China smoke. Many young boys and girls have the habit of smoking, though they are middle school students.
    More and more people have come to realize how serious this problem is. But they are never bored with it. ①Some people think it is a kind of fashion, some think it is of great fun and others think that smoking can relax themselves.
    Smoking causes many illnesses. A lot of people always cough(咳嗽)②______ ______smoking. The most serious illness caused by smoking is lung cancer. At the same time smoking is a waste of money. Beside, careless smokers may cause dangerous fires.
    It's known that smoking does harm to our health. We should get rid of(摆脱) the bad habit of smoking.

下列运算正确的是

自行车是一种人们常用的、简便无污染的交通工具．仔细观察自行车的构造（图），想想你的使用过程，结合所学有关知识，请提出一个与简单机械知识有关的问题，并针对提出的问题作出简要的回答．

问题：____ ？

回答：____ ．