语法总结
should have done / ought to have done
这两个结构都表示“本来应该做却没有做”，课文里用来委婉批评 Jimmy 的路线选择。
相似句：
- You should have told me the truth earlier.
- He ought to have taken a taxi instead.
must 表推测
Jumbo must weigh a few tons 不是“必须重”，而是说话人很有把握的推测。
相似句：
- He must be tired after the long journey.
- That box must weigh at least twenty kilos.
as 表原因
as Jumbo was holding up the traffic 在文中表示“因为”，语气比 because 更平缓。
相似句：
- As it was raining, we stayed indoors.
- As he was late, he missed the start.