From the trachea, air enters the ) , which branch out into smaller tubes called ).

(1)
a. bronchi
b. nostrils
c. pharynx

(2)
a. alveoli
b. bronchioles
c. diaphragm

1. A and 2. B

Respiration is a cellular metabolic process. It is common in all living organisms found on earth. In humans the respiratory system is highly complex. It includes specialized organs, which facilitates the exchange of gases (oxygen and carbon dioxide) throughout the body. In humans the respiratory system includes organs like nasal passage, larynx, trachea, bronchial tubes, alveoli and lungs.

In the respiratory system, the air enters through the nostrils, then the air passes to the pharynx to larynx, then to trachea. The trachea transfers the air to the lungs. Inside the lungs  several branching tubes are present, which are called as bronchi. The bronchi further form network of small tubes called as bronchioles. The bronchioles are further branched into small blood vessels called as arteries. In arteries, the exchange of gases (carbon dioxide and oxygen) takes place.