如图所示，在做“寻找等效力”的实验中，在水平放置的木板上垫一张白纸，把橡皮条的一端固定在A点上，另一端连接两根细线，然后通过细线用两个互成角度的弹簧秤来拉橡皮条，使橡皮条的结点拉到某一点O，记录两弹簧秤的读数和两细线的方向以及结点O的位置；然后改用一个弹簧秤把橡皮条的结点再次拉到O点，记录弹簧秤的读数和细线的方向.请问：为什么两次要将结点拉到同一位置（　　）

C

    I'm a 34-year-old man. I lived in a nice house and have a successful career. But my life was not always so great. I had trouble learning from an early age. The school life was difficult for me.

    My life improved quickly when I discovered art. The art world gave me a chance to express myself without words. I went to an art school and became good at making things with dough. Here I learned my first important lesson: although I had trouble learning, I could still be smart and well express myself with dough. And I became confident from then on.

    I got my next lesson from rock climbing. It was a fun thing but I was frightened from the start. After about five years of climbing, I learned that if you fall in love with something and do it all the time, you will get better at it.

    Later I decided to learn how to read. Every day I practiced reading. After two hard years, I did well in it. And reading changed my life a lot.

    After learning art, rock climbing, and reading, now I've learnt that when we face difficulties, we should not give up. God closes a door for you, and will open another window for you.