下列属于合成纤维的是（　　）

定义：a是不为1的有理数，我们把 $\frac{1}{1-a}$ 称为a的差倒数．

如：2的差倒数是 $\frac{1}{1-2}=-1$ ，﹣1的差倒数是 $\frac{1}{1-\left(-1\right)}$ = $\frac{1}{2}$ ．

已知 $a_1=-\frac{1}{3}$ ，

      Traffic lights are very important for us. I think you know this. But do you know how they were born?
      Garrett A. Morgan was the father of traffic lights. He was an American. His family was very poor. He left school at the age of 14. He was clever. He always looked for better ways to do things. And he liked to make new things.
      At that time, every American wanted to have a car. The streets were full of cars. The traffic was really bad. “Why not put three lights at each corner ? They should be red_,green_,and yellow，” Morgan said , “ Let them tell cars to go or stop.” He made a timer (定时器)to change the lights.
     So we have traffic lights today .