single rolling on double pushing track

例句：

1. 1. The single most important factoris the quality of the product.
   最重要的因素是产品质量。
2. 2. He has a single goal inlife: to be happy.
   他一生中只有一个目标，那就是快乐。
3. 3. She had only oneticket for the concert.
   她只有一张音乐会的票。