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: Gynaecology & Obstetrics
[{'content': 'Incidence of preterm delivery in twin pregnancy (MP2008) is: \nSelect and explain. \nA. 25%, B. 50%, C. 75%, D. 100%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 50%', 'role': 'assistant'}]