<div><b>If 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 neutralize the acid formed by the action of organisms Z on sugary food, and hence tooth decay sets in.</b><b>(a) </b>What is W known as?<b>(b)</b> What is (i) X, and (ii) Y?<b>(c) </b>What are organisms Z?<b>(d) </b>State one way of removing layer W from the teeth.</div>