The boy wanted to ride his bicycle in th
题型：选择题
问题：
The boy wanted to ride his bicycle in the street, but his mother told him _______.
A．not to
B．not to do
C．not do
D．do not to
题型：选择题
电阻R1、R2与交流电源按照图甲所示方式连接，R1=10 Ω，R2=20 Ω。合上开关S后，通过电阻R2的正弦交变电流i随时间t变化的情况如图乙所示，则
A.通过R1的电流的有效值是
B.R2两端的电压有效值是12 V
C.交流电源的周期为2s
D.通过R1的电流随时间变化规律是
(A)