Air enters through the nostrils of the nose and is partially filtered by the nose hairs, then flows into the nasal cavity.

•The nasal cavity is lined with epithelial tissue, containing blood vessels, which help warmthe air; and secrete mucous, which further filters the air.

The correct answer is option 'D': Mucus and fine hairs in the nostrils.

Explanation:
The air we breathe in contains various particles, such as dust, pollen, and bacteria, which can be harmful to our respiratory system. To protect our lungs from these particles, the air passing through the nostrils in humans is filtered by mucus and fine hairs in the nostrils.

1. Mucus:
- Mucus is a sticky substance produced by the mucous membranes lining the respiratory tract.
- It helps to trap dust, pollen, bacteria, and other particles present in the air.
- The mucus contains antibodies and enzymes that help in killing bacteria and preventing infections.
- The cilia, which are tiny hair-like structures present in the respiratory tract, move in coordinated motions to move the mucus along with trapped particles towards the throat.
- From the throat, the mucus is either swallowed or expelled through coughing or sneezing.

2. Fine Hairs:
- The nostrils are lined with fine hairs called vibrissae.
- These hairs act as a physical barrier, preventing larger particles from entering the respiratory system.
- The vibrissae also help to filter out dust, pollen, and other particles present in the air.
- The trapped particles stick to the mucus, which is then moved by the cilia towards the throat for removal.

Combination of Mucus and Fine Hairs:
- The combination of mucus and fine hairs in the nostrils provides an effective filtration system for the air we breathe.
- The mucus traps smaller particles that may pass through the fine hairs.
- The fine hairs prevent larger particles from entering the respiratory system.
- Together, they work to filter out harmful particles and prevent them from reaching the lungs.

In conclusion, the air passing through the nostrils in humans is filtered by mucus and fine hairs. The mucus traps smaller particles, while the fine hairs act as a physical barrier to prevent larger particles from entering the respiratory system. This filtration system helps to protect our lungs from harmful particles present in the air.