在一般情况下，城市主干道上的车流速度 $v$ （单位：千米/小时）是车流密度 $x$ （单位：辆/千米）的函数。当主干道上的车流密度达到200辆/千米时，造成堵塞，此时车流速度为0千米/小时；当车流密度不超过20辆/千米时，车流速度为60千米/小时。研究表明：当 $20\le x\le 200$ 时，车流速度 $v$ 是车流密度 $x$ 的一次函数。

    Everyone likes living in a clean and comfortable environment．If the environment is bad，it will affect(影响)our bodies，and make us not feel well．Sometimes we may be terribly ill．At mat time we don't want to work，and we have to stay in bed and rest at home．So the environment is very important to us．

    It's germs(微生物)that makes us ill．There are germs everywhere．They are very small and you can't find them with your own eyes．but you can see them with a microscope(显微镜)．They are very small and there may be hundreds of them on a very small thing，Germs can always be found in the dirty water．When we look at dirty water under the microscope．we shall see them in it．Germs can also be found in air and dust．If you cut your finger,some of the dust from the floor may go into it，and you will have much pain in it．Sometimes me germs will go into all of your body，and you will have pain everywhere．

    To keep us healthy，we should try to our best to make our environment become cleaner and tidier．This needs us to act together．

钛是一种硬度大、熔点高、常温下耐酸碱、耐腐蚀的金属，被誉为“未来的金属”。四川省内有大型钒钛磁铁矿，开采得的矿石经选矿后所得的钛铁矿可用于冶炼金属钛。钛铁矿的主要成分为钛酸亚铁（FeTiO₃）其中钛的化合价为（   ）

一个多边形的边长分别为2，3，4，5，6，另一个多边形和这个多边形相似，且最短边长为6，则最长边长为（　　）