Nitrous oxide is contraindicated in all except: (A) Vitreo-retinal surgeries, (B) Pneumothorax, (C) Mastectomy, (D) Laser surgery
Answer is C. Nitrous oxide is contraindicated in 

Pneumothorax, pneumoperitoneum
Vitreo-retinal surgery
Cochlear implant surgery
Intestinal obstruction
Laser surgery