This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'The tendency of colonic carcinoma to metastasize is best assessed by \nSelect and explain. \nA. Size of tumor, B. Carcinoembryonic antigen (CEA) levels, C. Depth of penetration of bowel wall, D. Proportion of bowel circumference involved.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]