用天平测量一个金属回形针的质量，可采用的方法是（  ）

本文原发表于____。作者冰心，原名____，我国现代作家。作品有小说《两个家庭》《斯人独憔悴》等，诗集有____、____，儿童作品有____。 

    This story happened three years ago. Jack was an eight-year-old boy at that time. He was a good pupil. He was good at every subject at school. But he could not get up early. He often slept until nine or ten o'clock in the morning. So he was often late for school.

    Jack's mother didn't want him to be late for school again. So she bought him a clock. She said to him,“ You must get up when you hear the clock ring.”

    “Yes, Mum.” said Jack. After that Jack got up at twenty to seven when he heard the clock ring.

    One day his mother forgot to make the clock ring at the usual time. And the next morning Jack didn't get up. It was six forty-two at that moment. It was time for breakfast. Mother went to see him. Jack was still in bed and his eyes were open.

    “Why didn't you get up?” Mother was angry.

    “You told me to get up when I heard the clock ring. So I am waiting for the bell.”

    “Mum, is the clock still sleeping?” Jack asked.