甲、乙两车在公路上，分别从P、Q两点同时同向运动，它们s﹣t图象的（路程﹣时间图象）如图所示，经过4s甲、乙相遇．甲、乙的速度分别为v_甲、v_乙，P、Q间的距离为s，则（　　）

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    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.