It is not proper to breathe through our nose because:
A.The dust particles would enter our bodies.
B.More carbon dioxide would be inhaled
C.Our throat will become sore
D.We will catch a cold easily.

Hint:The human nose having two inlets separated by a soft bony tissue is lined with mucous membrane. This mucous lining traps any dirt of dust particles that enters the nose along with the air that we breathe and hence restricts these dust particles from entering the lungs.

Complete answer:
When we breathe our nose, then air passes through the mucous lining into the sinus producing nitric oxide which helps the body’s muscles (in the heart and the blood vessels) to function. Hence we breathe through the mouth, there would be no mucous lining on the oesophagus to trap the dust particles, and instead the presence of saliva in the mouth would make them stick more to the mouth which may invite various diseases on the long run.

Hence, the correct answer is option A, the dust particles would enter our bodies.

Note:
Mouth-breathing occurs when we inhale and exhale through our mouths instead of our nose and this is most commonly observed when a person catches cold or when he or she is asleep. Common causes of mouth-breathing can be allergies, anatomical abnormalities such as blocked nose or sinus due to polyps, enlarged tonsils, a deviated septum, or a congestion from infection. Temporary mouth-breathing is not alarming but if it becomes chronic, then it will be a problem.