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
Earliest clinical sign of gingivitis is: 
Select and explain. 
A. Bleeding on probing, B. Change in colour, C. Change in contour, D. Change in size
A: Bleeding on probing
Answer: A: Bleeding on probing 
Explanation: None