题目

My brother Tom was very selfish when he was a little boy. He did not want share things with other people. For example，when he bought a chocolate cake, he put them in a secret place where I couldn’t find. Then he ate it all by himself. He never helped other. He said he is busy. That is, a game of tennis making him very busy. He did not care if something he did made people angrily. For instance, on one night he played strong and loudly music till four o’clock in the morning. But he is difference now. He often helps grandma with housework, helps mum with cooking and helps his classm试说出下列公式适用于任何电场的是 [　　] A．W=qUB．U=EdC．D．