一个微型吸尘器的直流电动机的额定电压为U，额定电流为I，线圈电阻为R，将它接在电动势为E，内阻为r的直流电源的两极间，电动机恰好能正常工作，则（   ）

    Bill,a 13-year-old boy, thought he had grown up to be a man. But his parents told him, "You won't be a real man until you begin to think about helping others."
    One moming, his parents gave him some money to buy some milk. Outside a shop he saw a homeless old~man who looked very sick. Bill went to him and asked. "what's wrong with you?"
    The old man answered, "I'm hungry,I haven't had any food for two days".
   “Let's go to the restaurant."
    When they got there, Bill asked the waiter to bring out bread and coffee to the old man. The old man finished the meal quickly. Afier the waiter took away the plate and the cup, the old man said, "Sorry for giving you too much trouble. I'm fine now. I'Il never forget your kindness! You're a very good young man."
    Bill was happy when he heard this. Just when he wanted to pay for the meal, the waiter came.To their surprise, Bill and the old man leamed that their food was free because it was the birthday of the boss, and they were the first customers that day.

二次世界大战以前，非洲三个独立的国家是____ ， ____ ， ____；在非洲东部有一条世界陆地最长的裂谷带 ____ ．

已知$x\in (b,a)$且$x\ne 0$，$\frac{1}{x}\in (\frac{1}{a},\frac{1}{b})$，则实数a,b满足（    ）