A liquid X of colour Y circulates in the human body only in one direction : from body tissues to the heart.

Among other things, liquid X contains germs from cells and dead cells. The liquid X is cleaned of germs and dead cells by a special type of white blood cells called Z. This cleaned liquid is then put into blood circulatory system in subclavian veins.

(a) What is (i) liquid X, and (ii) colour Y ?

(b) What are Z ?

(c) The liquid X is somewhat. similar to a component of blood. Name this component.

(d) Why is liquid X not red ? .