Correct Option
The correct option is (a).
Explanation
The human respiratory system is designed to facilitate gas exchange while protecting the lungs from harmful particulate matter and pathogens. The upper respiratory tract, specifically the nasal cavity, serves as the primary filtration and conditioning unit for inhaled air.
Statement-wise Analysis
- Statement 1 is Correct: The primary mechanism for air intake in humans is through the nostrils. Air passes through the nostrils into the nasal cavity, which is the first stage of the respiratory tract.
- Statement 2 is Incorrect: The nasal cavity is lined with fine hairs and mucus. These structures function as a biological filter, trapping dust particles, pollen, and other impurities present in the air to prevent them from reaching the lungs. They are highly effective at this filtration process.
- Statement 3 is Incorrect: Breathing through the mouth is generally not recommended because it bypasses the nasal cavity's protective functions. The nose warms, moistens, and filters the air, whereas the mouth does not provide these safeguards effectively. Mouth breathing can lead to the inhalation of cold, dry, and unfiltered air, which may irritate the respiratory tract.
Key Takeaway: The nasal cavity acts as a vital filtration system using hair and mucus to trap particulate matter; mouth breathing bypasses this defense and is therefore physiologically inferior to nasal breathing.