一、单项选择（30分）
从每小题A、B、C、D中选出一个能填入句中空白处的最佳答案。
1. _____I walked into class, the teacher had started teaching already.
A. By the time B. While
C. After D. Because
2. —Why don’t you have a notebook with you?
—I’ve_____ it at home.
A. lost B. forgotten C. left D. found
