Concept:Gas exchange in mammals occurs at the tiny air sacs where blood and inhaled air meet.Explanation:The lungs contain millions of small air sacs called alveoli.These alveoli have very thin walls and are surrounded by blood capillaries.Oxygen (O2) passes from the alveoli into the blood.Carbon dioxide (CO2) passes from the blood into the alveoli.This exchange happens in the alveoli, not in the bronchi or bronchioles.Answer:D. alveoli