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': 'Which one of the following investigations is considered to be "Gold standard" technique for diagnosis of arterial occlusive disease – \nSelect and explain. \nA. Doppler ultrasound blood flow detection, B. Duplex imaging, C. Treadmill, D. Digital substraction angiography (DSA)', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]