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: Dental
[{'content': 'Earliest clinical sign of gingivitis is: \nSelect and explain. \nA. Bleeding on probing, B. Change in colour, C. Change in contour, D. Change in size', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]