What are the two actions the receiver of a PGP email message can perform that allows establishment of trust between sender and receiver? 

Decode the message by decrypting the asymmetric key with his private key, then using the asymmetric key to decrypt the message. 

Decode the message by decrypting the symmetric key with his private key, then using the symmetric key to decrypt the message.  

Decode the message by decrypting the symmetric key with his public key, then using the symmetric key to decrypt the message. 

Decrypt the message by encrypting the digital signature with his private key, then using the digital signature to decrypt the message.