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 adapt himself to the outside world after being institutionalized for such a long time.
C、He could not enjoy the pleasure of life with his family members.
D、He could not bear any more the painful sufferings from his incurable disease.