Which statement about capillaries is correct? A. Their walls are covered with cilia B. They warm and humidify inhaled air C. Their walls are only one cell thick D. They move mucus through the lungs

The correct answer is C. Their walls are only one cell thick.

Capillaries are the smallest blood vessels in the body. They are only one cell thick, which allows for the exchange of nutrients and waste products between the blood and the tissues. Capillaries are found throughout the body, but they are most concentrated in the organs and tissues that require a lot of oxygen and nutrients, such as the brain, heart, and muscles.

Option A is incorrect because capillaries do not have cilia. Cilia are tiny hair-like projections that are found on the cells that line the airways. Cilia help to move mucus and other particles out of the lungs.

Option B is incorrect because capillaries do not warm and humidify inhaled air. This is done by the nose and the lungs. The nose warms and humidifies inhaled air before it reaches the lungs. The lungs also have a layer of mucus that helps to trap particles and bacteria.

Option D is incorrect because capillaries do not move mucus through the lungs. This is done by the cilia.