a dirty mark on the rug which must

[boggiee]

Hello,

Incorrect: There is a dirty mark on the rug which must be removed.

This is from the book 'Common English Errors' by Rewa Bhasin. And I'd like to ask why it is incorrect. I think it is ambigious.

Thanks.

 

[velisarius]

As you say, it's ambiguous: to which noun does "which" refer?