根据短文理解，选择正确答案。

    Frederick was ill and went to the hospital. A doctor looked him over and said, "Well, Mr Green, you're going to get some injections(针), and you'll feel much better. A nurse will come and give you the first this evening, and you'll have to get another one tomorrow evening.'' In the evening a young nurse came to Frederick's bed and said to him. "I'm going to give you your first injection now, Mr Green. Where do you want it?"

    The old man was surprised. He looked at the nurse for a while, then he said, "Nobody has ever let me choose that before. Are you really going to let me choose now?"

    "Yes, Mr Green," the nurse answered. She was in a hurry, "Where do you want it?"

    "Well, then,'' the old man answered with a smile, "l want it in your left arm, please."

汉武帝颁布推恩令，允许诸王将自己的封地分封给子弟，建立较小的侯国，主要是为了（　　）