阅读表达 (阅读短文，并根据短文内容回答问题。)

    Computers are useful machines．They can help people a lot in their daily life ． For example，they can help people to save much time to do much work，and they can help people to work out many problems easily．Our country asks everyone to learn to use computers before the twenty-first century，except the old people．

    Today more and more families own computers ． Parents buy computers for their children．They hope computers can help them improve their studies in school．Yet，many of their children use computers to play games，to watch videos or to chat．Computers can not help children to study but make them fall behind．So computers are locked in boxes by parents．

    In some other countries，even some scientists also hate computers．They say computers let millions of people lose their jobs or bring them a lot of trouble．Will computers really bring trouble to people or can they bring people happiness? It will be decided by today's students themselves!

在N₂+3H₂2NH₃的反应中，在5s中NH₃的浓度变化了8mol/L．则NH₃的平均反应速率（　　）

现有电源、开关、待测电阻R_x各一个，导线若干，另有下列4组备用器材，你认为选用后测不出未知电阻的是（　　）

下列说法中不正确的是（）