1. by chance _____        
2. by accident _____
3. by mistake _____                  
4.  by luck_____
5. by design _____
6. by good fortune_____
7. by necessity_____
8. I met an old friend of mine _____  in the street.
  我在街上偶然碰到了老朋友.
9. — How did you find your lost bike?
        你如何找到的你丢失的自行车?
    — _____.
         凑巧找到的.
10. I had no trouble passing on the driving test in the end ___.
      由于运气好最后我通过了驾照考试.
11. He has finished his task ahead of time______.
      由于进行了周密的筹划,他提前完成了任务.
12. The war will break out between the United Sates and Iran_____ in time.
      伊美之间迟早会不可避免地爆发战争.