How much did you __________ on the books?[ ]

A. cost

B. take

C. pay

D. spend

如图所示的电路，电源电动势为E，内阻为r，闭合开关S，滑动变阻器滑片向左移动，下列结论正确的是(　　)

A．电流表读数变小，电压表读数增大

B．小灯泡L变亮

C．电容器C上电荷量减小

D．电源的总功率变大