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': 'It can be stated that the superior haemorrhoidal veins —a) Drain into the inferior mesentric veinb) Have no valvec) Leave the anal canal at the pactinate lined) Cause external haemorrhoids \nSelect and explain. \nA. ac, B. a, C. ab, D. bc', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]