They have made a voluntary ________ to the cause of rural economic improvement.[ ]

A. donation  

B. congratulation

C. distribution  

D. contribution

电容器在充电和放电过程中，充放电电流与（）成正比。

A、电容器两端电压

B、电容器两端电压的变化率

C、电容器两端电压的变化量

D、与电压无关