Which of the following is true about an email that was signed by User A and sent to User B?

Which of the following is true about an email that was signed by User A and sent to User B?

A.
User A signed with User B’s private key and User B verified with their own public key.

B.
User A signed with their own private key and User B verified with User A’s public key.

C.
User A signed with User B’s public key and User B verified with their own private key.

D.
User A signed with their own public key and User B verified with User A’s private key.