以下三题均出自于新概念2的课后练习，看了答案仍然十分困惑， 还请老师帮忙解答。

1. When he had killed the guard, he dragged him into the bushes.

_____________ the guard he dragged him into the bushes. 

A. On having killed     B. When killing    C. When having killed      

答案：A     

为什么不能用C呢？

因为书中另一课有明确指出After having been instrued to drive out of town.等于After I had been instrued to drive out of town.  同理，When having killed 应该也等于When he had killed不是吗？ 

不选B是因为不能体现动作发生的先后顺序吗？

2. Realizing that this was no ordinary fish, the fisherman made every effort not to damage it in any way. 

When_____________that this was no ordinary fish, he tried not to damage it.

A.realizing     B. having realized     C. he realized   

答案：C

为什么不能选A或B？

3. When it was eventually brought to shore, it was found to be very long.

_____________eventually brought to the shore, it was found to be very long.

A. Been        B. When being        C. On being      

答案：C

为什么不能选A或B？