Read the following four statements carefully.

 i. Ventral respiratory group of medulla oblongata neurons can cause inspiration and expiration.

 ii. The part of the respiratory system starting with the external nostrils up to the terminal bronchioles constitutes the respiratory or exchange part of the respiratory system.

 iii. During swallowing epiglottis can be covered by a thin elastic cartilaginous flap called glottis to prevent the entry of food into the larynx.

 iv. Binding of oxygen with haemoglobin is primarily related to partial pressure of O₂.

Which of the above two statements are correct?

The correct statements are 1 and 4, hence the correct answer is option D.

Both inspiratory and expiratory neurons are present in the ventral respiratory group of neurons. When breathing is strong and deep, the ventral respiratory group of neurons is active; yet, when breathing is calm and serene, they remain inactive. 

Each haemoglobin molecule has a maximum oxygen carrying capacity of four atoms. However, there are reasons why the haemoglobin's affinity towards oxygen can lessen, one of which is the presence of carbon dioxide, other is the amount of oxygen/partial pressure of oxygen is equally important for its binding with the haemoglobin. 

Statements 2 and 3 are incorrect because of the following reasons:
- The respiratory system's directing component is established by the outer nostrils and extends to the terminal bronchioles, while the respiratory or trade part is built by the alveoli and their ducts.
- A small, flexible cartilaginous fold known as the epiglottis can be used to secure the glottis during gulping in order to prevent food from entering the larynx.

Hence the correct answer is option D.