At which thoracic vertebra does the trachea divide into right and left primary bronchi?

$A$ fluid-filled double-membranous layer surrounds the lungs. Name it and mention its important function.

Identify the component of the respiratory system which displays the features given below and choose the correct option:
$I.$ Double layered
$II.$ Fluid contained in it reduces the friction on the lung surface
$III.$ Its outer layer is in contact with the thoracic wall
$IV.$ Its inner layer is in contact with the lungs

Which is the narrowest and most delicate tube in the lungs?

Which structure opens into the alveolar sac?

The nasal cavity contains a large number of which of the following cells?