When we inhale oxygen, it is eventually transferred from the respiratory system to the circulatory system. the is where gas exchange takes place between the lungs and the blood.
a. alveoli
b. bronchi
c. vena cava
d. bronchioles?

I believe the answer is A, the alveoli. Alveoli are numerous structures found in the lungs that have features that help them to undertake gaseous exchange in the respiratory system. This exchange takes place in the alveoli in the lungs and the capillaries that envelop them, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.

Try and make the question more clear : )

It explains why rocks appear in layers