Which of the following statements is true?

a. A file encrypted with X’s private key can only be decrypted by using X’s private key.

b. A file encrypted with X’s private key can only be decrypted using X’s public key.

c. A file encrypted with X’s private key can only be decrypted by using Y’s private key.

d. A file encrypted with X’s private key can only be decrypted using Y’s public key.