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: Social & Preventive Medicine
[{'content': 'Complete contraction of a family starts when \nSelect and explain. \nA. Birth of last child, B. Last child leaves the home, C. 1st Spouse dics, D. Death of last child', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]