## Core Concept
A Primary Health Centre (PHC) is the basic structural and functional unit of the public health services in developing countries. The population covered by a PHC varies depending on the terrain, with hilly or tribal areas having a smaller population due to accessibility issues.

## Why the Correct Answer is Right
In hilly regions, PHCs are established to cover a population of 30,000, considering the difficult terrain and lower population density. This is in contrast to plain areas where a PHC typically covers a population of 40,000. The reduced population coverage in hilly areas ensures that healthcare services remain accessible to the population.

## Why Each Wrong Option is Incorrect
* **Option A:** 40,000 - This is the population covered by a PHC in plain areas, not hilly regions.
* **Option B:** 20,000 - This is lower than the specified population for hilly areas.
* **Option D:** 80,000 - This is the population covered by a Community Health Centre (CHC), not a PHC.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the population covered by a PHC varies by region: 30,000 in hilly or tribal areas and 40,000 in plain areas. Understanding these differences is crucial for planning and delivering healthcare services effectively.

## Correct Answer: B. 30,000.