Dorian is sending a digitally signed email to Poly. With which key is Dorian signing this message and how is Poly validating it?

A. Dorian is signing the message with his public key, and Poly will verify that the message came from Dorian by using Dorian's private key.
B. Dorian is signing the message with Poly's private key, and Poly will verify that the message came from Dorian by using Dorian's public key.
C. Dorian is signing the message with his private key, and Poly will verify that the message came from Dorian by using Dorian's public key.
D. Dorian is signing the message with Poly's public key, and Poly will verify that the message came from Dorian by using Dorian's public key.