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': 'In colour doppler the color depends upon: \nSelect and explain. \nA. Strength of returning echo, B. Relation of transducer to blood flow, C. Frequency of doppler used, D. Type of doppler machine used', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Relation of transducer to blood flow', 'role': 'assistant'}]