In which sentence is the word my used as an INTERJECTION?

A. My letter to the editor was published today
B. I expressed my ideas about the new freeway.
C. I could not believe my eyes.
D. My, there were a lot of letters on the subject!

D. My, there were a lot of letters on the subject!

The word "my" is used as an INTERJECTION in sentence D. "My, there were a lot of letters on the subject!"