In the digital signature mechanism, the sender encrypts the data file with his own private key to generate a digital signature; The receiver receives this digital signature and decrypts it using the sender's public key. If the decryption result is the same as the summary result generated by the original data file, it means that the file was sent by the sender; Otherwise, it is considered that the file was sent by a forged third party. According to the meaning of the question, you should choose a.