A Compound can best be described as: Same atoms bonded together.
At least two different atoms bonded together.
O A cell membrane with a bi-phase layer.
A blend of different molecules.

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Air enters through the nose (and sometimes the mouth), moves through the nasal cavity, the  pharynx, the  larynx, enters the  trachea, moves through the  bronchi  andbronchioles  till the alveoli.