If the radius of dentinal tubule is halved, the flow rate will decrease by:: (A) 2 times, (B) 4 times, (C) 8 times, (D) 16 times
Answer is D. Flow rate is a function of tubule diameter,pulpal pressure difference,viscosity and tubule length. 
A two fold reduction which means making it half, results in a 16 fold reduction as per the formula.