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
The mechanism by which ergometrine stops postpartum hemorrhage is that it: 
Select and explain. 
A. Causes vasoconstriction of uterine arteries, B. Increases tone of uterine muscle, C. Promotes coagulation, D. Induces platelet aggregation
B: Increases tone of uterine muscle
Answer: B: Increases tone of uterine muscle 
Explanation: None