阅读下面短文，然后从各题所给的四个选项中选出一个最佳选项。

    My mother only had one eye. And because of this, I never wished her to show up in my  1  , being afraid that my classmates would find out I had an ugly mother.

One day during elementary school, I was terribly ill. My mother came.     "Your mom only has one eye?!" asked some of my classmates. I was so  2 .     I wished my mother would just disappear from this world. "If you make me teased(嘲笑)，why don't you just die?" I shouted at her, taking no notice of the sad look on her face. My mother just handed me some  3and left without saying    4   .     At that time, I felt  5  to say what I always wanted to say, and I didn't think I had hurt her feelings very much. That night I saw my mom      6  in her room, so quietly, as if she was afraid that she might   7  me. Even so, I hated her tears from one eye. I made a   8  : I must study hard and leave my mother.     Years later my dream came true. I was quite successful and lived  9 . I never thought of going back to see my "ugly" mother  10 one day I got a letter, which said, "My son... I'm sorry I only have one eye. When you were little, you got into a(n)  11   and lost your eye. As a mom, I couldn't   12  watching you live with only one eye.  13   I gave you mine. I was never  14   with you and I never regretted(后悔) what I did because I    15   you…"     I cried out aloud. Only then did I realize how beautiful my mother was!

将两个质量均为m的小球a、b用细线相连后，再用细线悬挂于O点，如图所示．用力F拉小球b，使两个小球都处于静止状态，且细线Oa与竖直方向的夹角保持θ=60°，则F的最小值为（   ）

衣着素净（   ）