In The Shawshank Redemption, why did the old inmate, Book, kill himself after he was released from the prison?

A、He could not find a decent job and earn enough to support himself after being released from the prison.
B、He could not bear any more the painful sufferings from his incurable disease.
C、He could not adapt himself to the outside world after being institutionalized for such a long time.
D、He could not enjoy the pleasure of life with his family members.