语法总结
被动语态
can be heard、was built、been driven away、be knocked down 都突出动作承受者。
相似句：
- The noise can be heard everywhere.
- The old house was knocked down.
must have done 表示对过去的推测
Over a hundred people must have been driven away 表示根据噪音作出的肯定推测。
相似句：
- He must have missed the bus.
- They must have left early.