John wants to send Marie an email that includes sensitive information, and he does not trust the network that he is connected to. Marie gives him the idea of using PGP. What should John do to communicate correctly using this type of encryption?

A. Use his own private key to encrypt the message.
B. Use his own public key to encrypt the message.
C. Use Marie's private key to encrypt the message.
D. Use Marie's public key to encrypt the message.