a piece

例句：

1. 1.I need a piece of paper to write on.
   我需要一张纸来写字。
2. 2. She gave me a piece of cake as a treat.
   她给了我一块蛋糕作为奖励。
3. 3. He cut a piece of meat from the roast.
   他从烤的肉里切了一块。