Which of the following statements are true of the paranasal sinuses? (Read carefully and select all of the correct statements.)

A)They are lined with simple squamous epithelium that is smooth to prevent friction.
B)They are air cavities in the frontal and parietal bones.
C)If they fill with mucus, they may become painful.
D)An upper respiratory pathogen may infect these sinuses.
E)They make the skull lighter in weight.
F)They provide more air to vibrate when we speak.