The correct answer is D. All of the above.
Mucus is a thick, sticky substance that is produced by the cells in the lining of the nose. It helps to protect the lungs by trapping dust, dirt, and other particles that are breathed in. It also helps to keep the air passages clear by trapping mucus and other debris. Additionally, mucus helps to protect the nose from infection by trapping bacteria and viruses.
The mucus in your nose is made up of water, salt, and proteins. It also contains cells that produce antibodies, which help to fight infection. The mucus is constantly being produced and then swallowed or blown out of the nose.
The amount of mucus produced by the nose can vary depending on a number of factors, including the weather, allergies, and illness. When you have a cold or the flu, your body produces more mucus in an effort to fight off the infection. This can cause your nose to run and feel stuffy.
If you are experiencing excessive mucus production, it is important to see a doctor to rule out any underlying medical conditions.