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 contrast in X-rays is dependent - \nSelect and explain. \nA. Kvp, B. Mas, C. Duration of exposure, D. Distance between source and object', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Kvp determines\xa0\xa0\xa0→\xa0 Penetrating power of X-ray, contrast.\nmAs determine\xa0\xa0\xa0→\xa0 Number of X-ray photons, density of film (Blackening of film).', 'role': 'assistant'}]