语法总结
must have been done
must have been found 表示对过去被动事实的肯定推测。
相似句：
- The wallet must have been picked up.
- The letter must have been sent yesterday.
which 引导定语从句
a note which said... 中 which 指 note，并引出纸条上的内容。
相似句：
- He found a letter which explained everything.
- I read the note which she left.