Able wants to send a file to Baker over the Internet and protect the file so that only Baker can read it and can verify that it came from Able. What should Able do?

a. Encrypt the file using Able's public key, and then encrypt it again using Baker's private key. 

b. Encrypt the file using Able's private key, and then encrypt it again using Baker's private key.

c. Encrypt the file using Able's public key, and then encrypt it again using Baker's public key. 

d. Encrypt the file using Able's private key, and then encrypt it again using Baker's public key.