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
During heavy exercise the cardiac output (CO) increases upto five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by 
Select and explain. 
A. Increase in the number of open capillaries, B. Sympathetically mediated greater distensibility of pulmonary vessels, C. Large amount of smooth muscle in pulmonary arterioles, D. Smaller surface area of pulmonary circulation
A: Increase in the number of open capillaries
Answer: A: Increase in the number of open capillaries 
Explanation: In lungs, there are inactive capillaries that open up during increased blood flow, as in case of rigorous exercise. Along with that, the flow rate through each capillary is also increased. So when there is an increased systemic BP, the pressure in pulmonary circulation is raised only by a small level.