At which level of thoracic vertebra does the trachea divide?

The trachea divides into left and right primary bronchi at the level of $-$

Vocal cords are present in . . . . . . .

The figure shows a diagrammatic view of the human respiratory system with labels $A, B, C$,and $D$. Select the option which gives the correct identification and main function and/or characteristic.

The lungs are composed of $........$.

$A$ thin-elastic cartilaginous flap which prevents the entry of food into the larynx is known as