Single circulation,i.e.,blood flows through the heart only once during one cycle of passage through the body,is exhibited by:

$(a)$ Draw a sectional view of the human heart and label on it Aorta,Pulmonary arteries,Vena cava,Left ventricle.
$(b)$ Why is double circulation of blood necessary in human beings?

$(a)$ Name the following:
$(i)$ Part where air is filtered by fine hair and mucus.
$(ii)$ Part which terminates in balloon-like structures.
$(iii)$ Balloon-like structures where exchange of gases takes place.
$(iv)$ Part which separates the chest cavity from the abdominal cavity.
$(b)$ Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms?

The blood leaving the tissues becomes richer in

Two green plants are kept separately in oxygen-free containers,one in the dark and the other in continuous light. Which one will live longer? Give reasons.

Choose the forms in which most plants absorb nitrogen.
$(i)$ Proteins
$(ii)$ Nitrates and Nitrites
$(iii)$ Urea
$(iv)$ Atmospheric nitrogen