Explain why, when air is taken in and let out during breathing, the lungs always contain a residual volume of air.

An organism $X$ having breathing organs $A$ lives on land. When organism $X$ goes underwater, it cannot survive for a long time unless carrying an oxygen cylinder. On the other hand, the organism $Y$ having breathing organs $B$ always lives in water and if taken out of water, it dies after a short while. A third organism $Z$ having breathing organs $C$ and $D$ which lives on the banks of ponds, lakes, and rivers can survive on land as well as in water equally well.
$(a)$ What could organism $X$ be? Name the breathing organs $A$.
$(b)$ What could organism $Y$ be? Name the breathing organs $B$.
$(c)$ What could organism $Z$ be? Name the breathing organs $C$ and $D$.
$(d)$ Out of $X$, $Y$, and $Z$, which organism is:
$(i)$ Amphibian,
(ii) Aquatic,
(iii) Terrestrial?

$(i)$ Under normal conditions, what is the rate of breathing per minute for an adult? (ii) Why does the rate of breathing increase significantly during vigorous exercise?

Name the substance whose build-up in the muscles during vigorous physical exercise may cause cramps.

The process of respiration is:

Where does aerobic respiration occur? How many molecules of $ATP$ are released during the complete oxidation of one glucose molecule?