Explanation:

The correct answer is option 'B' - contraction of diaphragm.

The process of inspiration, also known as inhalation, involves the intake of air into the lungs. It is an active process that requires the contraction of certain muscles and the expansion of the thoracic cavity to create a pressure difference between the lungs and the external environment. This pressure difference allows air to flow into the lungs.

Process of Inspiration:

1. Contraction of Diaphragm:
- The diaphragm is a dome-shaped muscle located at the base of the thoracic cavity.
- During inspiration, the diaphragm contracts and moves downward, flattening its shape.
- This contraction increases the volume of the thoracic cavity vertically.
- The diaphragm plays a crucial role in expanding the lungs and creating the necessary pressure difference for air to enter.

2. Relaxation of External Intercostal Muscles:
- The external intercostal muscles are located between the ribs and are responsible for elevating the ribcage.
- During inspiration, these muscles contract, causing the ribcage to move upwards and outwards.
- This action further increases the volume of the thoracic cavity horizontally.

3. Increase in Volume of Thoracic Cavity:
- The combined actions of the diaphragm and external intercostal muscles result in an increase in the volume of the thoracic cavity.
- As the volume of the thoracic cavity increases, the pressure within the lungs decreases.
- This decrease in pressure creates a pressure gradient, with the atmospheric pressure being higher than the pressure within the lungs.

4. Inflow of Air:
- Due to the pressure difference, air flows from an area of higher pressure (atmosphere) to an area of lower pressure (lungs).
- The air enters through the nose or mouth, passes through the pharynx, larynx, trachea, and bronchi, eventually reaching the alveoli in the lungs.

In conclusion, the first step in the process of inspiration is the contraction of the diaphragm, which increases the vertical dimension of the thoracic cavity and creates the necessary pressure difference for air to flow into the lungs.

Ans is b because contraction of diaphragm increase int ra pulmonary volume for intake of o2 rich air