Internet is educational . Many people think children should ____to surf the Internet as they wish . In this way ,they can look____ different subjects on different kinds of websites according to their own interest . But in my opinion ,children should always be watched____surfing the internet .

    Many websites have information and pictures that children should not see .There are many things that children don't understand .All of these may have some bad influence on children's ____ Also , it is dangerous to let childern use chat rooms by themselves .Many parents tell their children not to talk to ____on the street , so they should be careful of ____their children talk to on the internet,too . What's ____ sitting in the front of a computer for a long time isn't good for children's eyes , so ____should monitor(监视) the time that children spend on the internet .

    In a word ,children should ____ away from anything that could be bad for them . I ____believe that it is necessary for parents to watch their children when they use internet .

${\left(5x-\frac{1}{\sqrt{x}}\right)}^4$展开式中x的系数为 (　　)

沿固定斜面下滑的物体受到与斜面平行向上的拉力F的作用，其下滑的速度﹣时间图线如图所示．已知物体与斜面之间的动摩擦因数为常数，在0～5s，5～10s，10～15s内F的大小分别为F₁、F₂和F₃，则（   ）