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:

• A. Large amount of smooth muscle in pulmonary arterioles
• B. Smaller surface area of pulmonary circulation
• C. Increase in the number of open capillaries
• D. Sympathetically mediated greater distensibility of pulmonary vessels