This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Physiology
Small airways have laminar air flow because 
Select and explain. 
A. Reynold's number>2000, B. Very small diameter, C. Extremely low velocity, D. Low cross sectional area.
C: Extremely low velocity
Answer: C: Extremely low velocity 
Explanation: The probability of turbulence is determined by Reynold's number. When it is more than 3000, the flow is almost always turbulent. When it is less than 2000, the flow is usually non-turbulent (laminar). Reynold's number is directly propoional to Density of fluid Diameter of vessel Velocity of flow Ref: guyton and hall textbook of medical physiology 12 edition page number:321,321,322