下列说法中正确的是（　　）

A．通过导体的电压是形成电流的原因

B．电源是产生电压的必要条件

C．导体中有大量的自由电荷，只要构成通路，导体中就会有电荷通过

D．电路中只要有电压，就会有电流

填空题。

Peter Judd joined the army   1  he was eighteen, and for several months he     2  (teach) how to be a

good soldier. He did quite well in everything   3  shooting. One day he and his friends were practicing   4   (shoot), all of _5_were doing quite well except Peter. After he had shot at the target nine times

and had not hit it once, the officer  6   was trying to teach the young soldiers to shoot said, "You're

quite   7   (hope). Peter! Don't waste your last bullet too! Go behind that wall and shoot yourself with it!"

     Peter felt ashamed. He went behind the wall, and a few seconds later the officer and the   8   young

soldiers heard the sound of a shot.

     "Heavens!" the officer said, "Has that silly man really shot  9  ?" He ran behind the wall  10   (anxious), but Peter was all right. "I'm sorry, sir." he said, "but I missed again."