posted by Asia .
which one of the sentences uses but as a preposition?
A I wanted to leave,but I was embaressed to do so.
B The lake is pretty, but it's too cold for swimming.
Neither of those sentences uses "but" as a preposition. In both it is used as a conjunction.
"But" is very seldom used as a preposition. When it is used as a preposition, "but" means the same as except. Example: No one but Andrew ate the worms.
I NEED THE NINE PROOFREADING MARKS
I don't know which nine you need, but here is a good comprehensive webpage: