阅读短文，下列问题

 B

American boys and girls love to watch television. Some children spend six hours a day in school and four to six hours a day in front of the television set. Some even watch television for eight hours or more on Saturday.

     Televisions are like books or films. A child can learn bad things and good things from them. Some programmes (节目) help children to understand (理解)the news, others show people and places from other countries or other time in history. With television a child does not have to go to the zoo to see animals to the ocean to see a ship. Boys and girls can see a play, a concert or a game at home.

     Television brings many places and events into our homes. Some programmes show crime (犯罪) and other things that are bad for children, so parents sometimes help them to find other interesting things to do.

玩滑板是中学生喜爱的运动．用脚蹬地后站上滑板也能继续向前滑行，是因为滑板具有____ ．小明提着质量为0.6kg的滑板沿水平路面走了50m，此过程中他向上提滑板的力对滑板做功　____ 　J．

在真空中的光滑水平绝缘面上有一带电小滑块。开始时滑块静止。若在滑块所在空间加一水平匀强电场E₁，持续一段时间后立即换成与E₁相反方向的匀强电场E₂。当电场E₂与电场E₁持续时间相同时，滑块恰好回到初始位置，且具有动能E_k。在上述过程中，E₁对滑块的电场力做功为W₁，电场力大小为F₁；E₂对滑块的电场力做功为W₂，电场力大小为F₂则（     ）