This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neck Triangles and parotid gland, Subject name: Anatomy
Posterior boundary of Carotid Triangle is formed by 
Select and explain. 
A. Superior belly of omohyoid, B. Posterior belly of digastric, C. Sterno hyoid, D. Sternocleido mastoid
D: Sternocleido mastoid
Answer: D: Sternocleido mastoid 
Explanation: Boundaries of carotid triangle IN FRONT & ABOVE- Post Belly of Digastric and Stylohyoid Muscle. IN FRONT & BELOW- Superior Belly of Omohyoid. BEHIND- Anterior Border of SternoCleidomastoid.