This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Surgery
[{'content': 'In doing phrenic nerve block, it is best to infiltrate \nSelect and explain. \nA. Scalenus anterior, B. Scalenus posterior, C. Posterior border of sternomastoid, D. Anterior border of sternomastoid', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Phrenic nerve is blocked 3 cm above the clavicle at the posterior border of sternomastoid.', 'role': 'assistant'}]