Sustained exercise can increase the blood flow rate of the heart by a factor of 5 with only a modest increase in blood pressure. This is a large change in flow. Although several factors come into play, which of the following physiological changes would most plausibly account for such a large increase in flow with a small change in pressure? 

A. A decrease in the viscosity of the blood 

B. Dilation of the smaller blood vessels to larger diameters 

C. Dilation of the aorta to larger diameter 

D. An increase in the oxygen carried by the blood