阅读下列短文，根据短文内容，从短文后的选项中选出适当的选项补全短文。
Alex Smith, a fifteen-year-old boy, studied in an American boarding school (寄宿学校). In this school,
students could go into town after school in the afternoon.
1_____ One afternoon, Alex walked to the town. He looked about at the shops and then went to a cinema.
2_____ It was eight o'clock. He was a little worried, He walked back to the school as fast as possible.
When he arrived, he ran quickly to the main entrance (入口). It was locked. He looked up at the window
of his dormitory. It was on the third floor. The window was open. 3_____ But the door was also locked.
Then he saw another open window on the first floor. It was the window of the teachers' office. 4_____ Alex
quickly climbed into the room. Just then, he heard someone opened the door. He had to hide under the sofa
at once. One minute later, a teacher came in. 5_____ Under the sofa Alex had to lie on the floor.
A. He looked into the room-no one was there.
B. After the film was over, he looked at his watch.
C. But they must go back to the school before six o'clock.
D. The teacher turned on the light, sat down on the sofa and began to read a book.
E. But it was quite dark and he went round the school building to another door.