-How do you think he managed to improve his English in such a short time?

-______ four hours on it every day．[ ]

A．Because of spending

B．To spend   

C．By spending

D．Spending 

加在二极管两端的电压和流过二极管的电流之间的关系叫二极管的（）。

A.伏安特性

B.稳压特性

C.稳流特性

D.放大特性