Which one of the following statements is incorrect?
$\mathrm{A}$. Trachea divides at the level of $5^{\text{th }}$ thoracic vertebra into a right and left primary bronchi.
$\mathrm{B}$. Initial bronchioles are supported by complete cartilaginous rings.
$\mathrm{C}$. Each terminal bronchiole gives rise to a number of very thin, irregular walled and vascularised baglike structure called bronchi.
$\mathrm{D}$. Larynx is a cartilaginous box which help in sound production and hence called the sound box.