Which statement about capillaries is correct?
a. their walls are only one cell thick
b. they move mucus through the lungs
c. their walls are covered with cilia
d. they warm and humidify inhaled air

Option a.

Explanation:

Cappilaries are thin, branched blood vessels that make network between venues and atrioles.

There main function is to transfer oxygen and nutrients from blood to other body part and to collect waste products from different tissues.

Cappilaries show presence of thin walls, made up of only one cell that help them to perform their function efficiently.

Thus, the correct answer is option). a.