This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiac Output, Subject name: Physiology
A 60-year old patient underwent renal aery doppler which shows narrowing and turbulence in right renal aery. If radius of the aery is reduced by 1/3rd, resistance to blood flow in right kidney would have increased by: 
Select and explain. 
A. 3 times, B. 9 times, C. 16 times, D. 81 times
D: 81 times
Answer: D: 81 times 
Explanation: "Fouh power law" of radius ~ Blood flow varies directly and resistance inversely with the fouh power of the radius (r 4). If the radius is changed by a factor of 3 (reduced by 1/3rd), resistance would change by (34).