The trachea is part of the respiratory tract. It is a tube that is about four inches long and less than one inch wide. The trachea contains about twenty C-shaped rings of cartilage. Muscle lines the lack of each ring. What is the MAIN purpose of these rings?

A) to give the trachea support
B) to give the trachea moisture
C) to make the trachea inflexible
D) to prevent obstruction of the trachea