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: Radiology
[{'content': 'The earliest sign of foetal life is best detected b: \nSelect and explain. \nA. Doppler, B. Rcal time ultrasound, C. Fetoscopy, D. X-ray', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Rcal time ultrasound', 'role': 'assistant'}]