阅读下面短文，从每题所给的A、B、C、D四个选项中，选出能正确回答所提问题或完成所给句子的最佳答案。
Sixteen years ago, a boy gave me an important gift. It was a smile.
It was the early autumn of my first year at a middle school, and my old school was far away. As a result, no one knew who I was. I was very lonely, and afraid to make friends with anyone.
Every time I heard the other students talking and laughing, I felt my heart break. I couldn't talk with anyone about my problems.
Then one day, my classmates were talking happily with their friends, but I was sitting at my desk unhappily as usual. At that moment, a boy entered the classroom. I didn't know who he was. He passed me and then turned back. He looked at me, with a smile.
Suddenly, I felt the touch of something bright and friendly. It made me feel happy and warm. That smile changed my life. I started to talk with other students and made friends. Day by day, I became closer to everyone in my class. The boy with the lucky smile has become my best friend now.
One day I asked him why he had smiled, but he couldn't remember doing so!
It doesn't matter because all the dark days have gone. I believe that the world is what you think it is. If you think it is lonely, you might always be alone. So smile at the world and it will smile back.
｝, N=｛
｝, 则 M
N = ( )