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
The non-invasive method to measure the blood flow is: 
Select and explain. 
A. Electric pulp test, B. Percussion, C. Radiograph, D. Laser Doppler flowmetry
D: Laser Doppler flowmetry
Answer: D: Laser Doppler flowmetry 
Explanation: None