Why are the rings of cartilage surrounding the trachea C-shaped?

A)The C rings of cartilage allow the trachea to change diameter during breathing.
B)The C rings of cartilage allow the esophagus to expand during swallowing.
C)The C rings of cartilage allow the esophagus to expand during swallowing,allow the trachea to change diameter during breathing,and keep the trachea patent (open).
D)The C rings of cartilage keep the trachea patent (open).