This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 1, Subject name: Physiology
According to Poiseuille's law, reducing the radius of an airway to one-third will increase its resistance how many fold? 
Select and explain. 
A. 3-Jan, B. 3, C. 9, D. 81
D: 81
Answer: D: 81 
Explanation: Poiseuille's law states that during laminar flow, airway resistance is inversely propoional to the 4th power of the radius, other things being equal. Therefore, a reduction in the radius by a factor of 3 increases the resistance by 34, that is, 81.