E-mail message authenticity and confidentiality is BEST achieved by signing the message using the: 
A、sender's private key and encrypting the message using the receiver's public key. 
B、sender's public key and encrypting the message using the receiver's private key. 
C、receiver's private key and encrypting the message using the sender's public key. 
D、receiver's public key and encrypting the message using the sender's private key.