题目

如图所示，在绝缘的斜面上方存在着沿水平向右的匀强电场，斜面上的带电金属块沿斜面滑下。已知在下滑的过程中，金属块动能增加了12J，金属块克服摩擦力做功8J，重力做功24J，下列判断中正确的是       （    ）        A．金属块带负电        B．金属块克服电场力做功8J        C．金属块的机械能减少12J        D．金属块的电势能减少4J语法专练：用括号内所给单词的适当形式填空【1】I _______ (know)the Englishman for two years.【2】Her grandpa _______ (die)when she was 20.【3】He _______ (not come)yet. What do you think has happened to him?【4】We _______ (go)to school by bike every day.【5】So far I _______ (be)to many places in China.