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.

身边榜样 一起同行 
在某校，一提起九年级的李帅，同学们没有一个不竖起大拇指的。作为班长，他学习成绩优异，认真完成分管的工作，同学们不管在学习上或生活上有了困难，他总能够抢先去帮助。别看他一脸的书生气，面对不良风气却毫不畏惧。一天，当他发现在学校门口有个书贩正向学生兜售黄色书刊时，随即拨打了110报警电话，于是不法书贩当场被抓。假期里，李帅又参加了青年志愿者活动，利用这个假期做了一些有意义的事情。班委换届时，好朋友主张为他拉拉选票，被李帅严厉拒绝。同学们非常佩服他，选举时纷纷投了赞成票。
请回答：李帅为什么会赢得同学们的“赞成票”？ 

Hurry up. There's _____ time left.

一钢瓶中贮有质量m，密度为ρ的氧气，当用去质量为m/4的氧气后，钢瓶内所剩氧气的密度为   （     ）