Which of the following statements about the response of arteriolar smooth muscle to changing oxygen partial pressures is true?
a. both systemic and pulmonary arterioles respond to a decrease in po₂ by constricting.
b. both systemic and pulmonary arterioles respond to a decrease in po₂ by dilating.
c. systemic arterioles respond to a decrease in po₂ by dilating, but pulmonary arterioles constrict in response to decreased po₂.
d. systemic arterioles respond to a decrease in po₂ by constricting, but pulmonary arterioles dilate in response to decreased po₂.
e. changes in po₂ do not affect arteriolar smooth muscle in the pulmonary system.