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
[{'content': '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: \nSelect and explain. \nA. Increase in the number of open capillaries, B. Sympathetically mediated greater distensibility of pulmonary vessels, C. Large amount of smooth muscle in pulmonary aerioles, D. Smaller surface area of pulmonary circulation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Increase in the number of open capillaries', 'role': 'assistant'}]