The gaseous exchange between blood and air does not occur in

What is the signet ring-shaped cartilage?

Which structure is seen in the trachea so that the respiratory tract remains open?

During swallowing,the glottis can be covered by a thin elastic cartilaginous flap called $A$ to prevent the entry of food into the larynx. The trachea is a straight tube extending up to the $B$ cavity,which divides at the level of the $5^{th}$ thoracic vertebra into right and left primary $C$. Choose the correct option for $A, B$,and $C$ from the given four options to complete the above statement with reference to the $NCERT$ textbook.

Food does not normally enter the trachea because during swallowing of food

What is the function of the cilia in the respiratory tract?