下列反应属于置换反应的是( )
A:2HgO 2Hg+O2↑
B:H2+CuO Cu+H2O
C:C2 H5OH+3 O2 2CO2+3 H2O
D:C+O2 CO2
一辆小汽车在平直的公路上行驶,如左图所示,右图是某同学利用照相机进行“频闪摄影”后描绘出的小汽车行驶的路程与时间关系图像,则下列说法正确的是 ( )
如图所示电路,电源电压一定。电阻R1的阻值为R2的2倍。当开关S1、S2都闭合时,电流表A1的示数为I1,电流表A2的示数为5I1。当开关S1、S2都断开时,电流表A1的示数为I1′。则I1′与I1之比为( )
根据课文内容填空
Daming has got a____. It ____ walk. And it can dance. One day, hey will do____. They will do the housework. They will help children____. But they____do our homework.