H the teeth are not cleaned regularly, they become covered with a sticky yellowish layer W of food particles and bacteria. Since layer W covers the teeth, the alkaline liquid X secreted by glands Y inside the mouth cannot reach the teeth surface to neutralise the acid formed by the action of organisms Z on sugary food, and hence tooth decay sets in.

(a) What is W known as?

(b) What is (i) X, and (ii) Y?

(c) What are organisms Z?

(d) State one way of removing layer W from the teeth.