Which of the following provides the GREATEST assurance of message authenticity? 
A、The prehash code is derived mathematically from the message being sent. 
B、The prehash code is encrypted using the sender's private key. 
C、The prehash code and the message are encrypted using the secret key. 
D、The sender attains the recipient's public key and verifies the authenticity of its digital certificate with a certificate authority.