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: Pharmacology
[{'content': 'The mechanism by which ergometrine stops postpartum hemorrhage is that it: \nSelect and explain. \nA. Causes vasoconstriction of uterine arteries, B. Increases tone of uterine muscle, C. Promotes coagulation, D. Induces platelet aggregation', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]