This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Abnormal labor, Subject name: Gynaecology & Obstetrics
[{'content': 'Risk of preterm delivery is increased if cervical length is less than : \nSelect and explain. \nA. 2.5 cm, B. 3.0 cm, C. 3.5 cm, D. 4.0 cm', 'role': 'user'}, {'content': "Answer: A \nExplanation: During pregnancy cervical length of <2.5cm and internal os of 1cm increases the chance of miscarriage Reference: Dutta's Textbook of Obstetrics 7th edition page no 169", 'role': 'assistant'}]