Which of the following ensures a sender's authenticity and an e-mail's confidentiality? 
A、Encrypting the hash of the message with the sender's private key and thereafter encrypting the hash of the message with the receiver's public key 
B、The sender digitally signing the message and thereafter encrypting the hash of the message with the sender's private key 
C、Encrypting the hash of the message with the sender's private key and thereafter encrypting the message with the receiver's public key 
D、Encrypting the message with the sender's private key and encrypting the message hash with the receiver's public key