Give reasons for the following:
$(a)$ Rings of cartilage are present in the trachea.
$(b)$ Mucus is secreted along with $HCl$ in the stomach.

(N/A) Rings of cartilage are present in the trachea to prevent it from collapsing when there is no air inside it,ensuring that the airway remains open for breathing.
$(b)$ $(i)$ Mucus protects the inner lining of the stomach from the corrosive action of $HCl$ under normal conditions.
$(ii)$ Mucus lubricates the food,making it smoother so that it can move easily through the alimentary canal.