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: Physiology
Blood flow through a vessel varies directly with 
Select and explain. 
A. Resistance, B. Viscosity, C. Difference of pressure, D. Length of vessel
C: Difference of pressure
Answer: C: Difference of pressure 
Explanation: Blood flow is directly proportional to
-        Pressure gradient
-        Radius
Blood flow is inversely proportional to
-        Viscosity
-        Length
-        Resistance