Why does absorption of digested food occur mainly in the small intestine?

During the respiration of an organism $A$,one molecule of glucose produces two $ATP$ molecules,whereas in the respiration of another organism $B$,one molecule of glucose produces $38$ $ATP$ molecules.
$(a)$ Which organism is undergoing aerobic respiration?
$(b)$ Which organism is undergoing anaerobic respiration?
$(c)$ Which type of organism,$A$ or $B$,can convert glucose into alcohol?
$(d)$ Name one organism which behaves like $A$.
$(e)$ Name two organisms which behave like $B$.

Why is the rate of breathing in aquatic organisms much faster than that seen in terrestrial organisms?

How do carbohydrates,proteins,and fats get digested in human beings?

$(a)$ Draw a sectional view of the human heart and label the following parts on it:
Pulmonary artery,right atrium,left ventricle,septum.
$(b)$ What do the following transport?
$(i)$ Xylem
$(ii)$ Phloem
$(iii)$ Pulmonary vein
$(iv)$ Vena cava

Name any two substances which are selectively reabsorbed from the tubules of a nephron.