The alveoli contain ..........

The thyroid is a cartilaginous plate in:

What is the signet ring-shaped cartilage?

In humans,sound is produced by which of the following?

Alveolus of lungs is lined by . . . . . . .

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.