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': "The source of HCG is : \nSelect and explain. \nA. Syncytiotrophoblast, B. Cytotrophoblast, C. Langhan's layer, D. Chorionic villi", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Syncytiotrophoblast', 'role': 'assistant'}]