This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Incidence of preterm delivery in twin pregnancy is \nSelect and explain. \nA. 25%, B. 50%, C. 75%, D. 100%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Incidence of preterm in twin pregnancy is 50% Mean gestational age for twins is 37weeks Causes of prematurity: Overdistention of uterus Hydromnios Premature rupture of membranes Ref: Dutta Obs 9e pg 193.', 'role': 'assistant'}]