题目

Most students at school hope that they can ________activities that appeal to them after class instead of just doing their homework. A．apply for                    B．participate in C．stick to                         D．focus on 答案：B如图所示的电路，一灯泡和一可变电容器串联，下列说法正确的是（　　）A．a、b端接稳恒直流电，灯泡发亮B．a、b端接交变电流，灯泡发亮C．a、b端接交变电流，灯泡发亮，且将电容器电容增大时，灯泡亮度增大D．a、b端接交变电流，灯泡发亮，在不改变交变电流有效值的情况下增大其频率，灯泡亮度增大