This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Demography and family planning, Subject name: Social & Preventive Medicine
[{'content': 'Mechanism of action of oral contraceptive containing a combination of oestrogen-progestin is \nSelect and explain. \nA. Inhibition of ovulation, B. A change in the cervical mucus, C. Inhibition of motility of the uterine tubes, D. Inhibition of motility of sperms', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The mechanism of action of the combined oral pill is mainly to prevent ovulation This is achieved by blocking the pituitary secretion of gonadotropins that is necessary for ovulation to occur Reference: Park&;s textbook of preventive and social medicine, 23rd edition, Page no: 501', 'role': 'assistant'}]