<prompt> During heavy exercise the cardiac output CO increases upto five fold while pulmonary aerial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by:Increase in the number of open capillariesSympathetically mediated greater distensibility of pulmonary vesselsLarge amount of smooth muscle in pulmonary aeriolesSmaller surface area of pulmonary circulation  <response>: A i.e. Increase in the number of open capillaries 