49. He goes _____ at 8 a.m. and comes _____ at 5 p.m. Today he is _____.

　 A. on duty; off duty; on night duty　　　　　　　　　　　 B. on his duty; off the duty; on night duty

　 C. to duty; from duty; at night duty　　　　　　　　　　　　　　 D. in duty; out duty; at night duty