Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: The cilia in the respiratory system are responsible for preventing dirt and mucus from entering the bronchial tubes and lungs. If the cilia are damaged, the person's bronchial tubes and lungs are most likely to
be filled with mucus and dirt.