The following changes take place in an athlete's body during a $100 \ m$ race. Which change occurs first?

If a person holds their breath after expiration for about $25 \ s$, would there be an occurrence of any exchange of respiratory gases in the lungs during this period? Explain.

The trachea divides into two tubes at its lower end. What is the name of these tubes?

Explain the three pathways of breakdown of glucose in living organisms.

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?

Explain the three pathways of the breakdown of glucose in living organisms.