Look, this is my city. There are many bikes and buses on the streets. Let's count. There are ____ bikes and ____ buses. Look here, there are also ____ taxis and ____ cars. My city is big and beautiful!
小刚晚上做作业时,把台灯的插头插入插座,闭合台灯开关后,他家所有的照明灯都突然熄灭,检查发现总开关已跳闸,故障可能是( )
如图所示,电动势为E、内阻为r的电池与定值电阻R0、滑动变阻器R串联,已知R0=r, 滑动变阻器的最大阻值是2r.当滑动变阻器的滑片P由a端向b端滑动时,下列说法中正确的是( )
如图所示为条形磁铁和电磁铁,虚线表示磁感线,则甲、乙、丙、丁的极性依次是( )