## Core Concept
Population explosion refers to an extremely rapid growth in the size of a population over a relatively short period, often resulting from a combination of high birth rates and low death rates. This concept is crucial in understanding demographic changes and their implications on resource allocation and economic development. The rate that defines population explosion can vary slightly depending on the context, but a commonly used threshold is a population growth rate of more than 2% per year.

## Why the Correct Answer is Right
The correct answer, 2% per year, is a widely recognized threshold for population explosion. This rate indicates a very rapid increase in population size, which can put significant pressure on a country's resources, infrastructure, and economy. A growth rate above 2% per year suggests that the population is doubling in less than 35 years, which is a rapid pace of growth that can be challenging to manage.

## Why Each Wrong Option is Incorrect
- **Option A:** 1% per year. This growth rate is considered relatively low and does not signify a population explosion. Most countries aim for a growth rate around or below 1% per year for sustainable development.
- **Option B:** 1.5% per year. While this is a moderate growth rate, it does not meet the threshold for what is typically considered a population explosion.
- **Option D:** 3% per year. Although this is indeed a high growth rate indicating rapid population increase, the question asks for the threshold that defines population explosion, which is more commonly cited as 2% per year.

## Clinical Pearl / High-Yield Fact
A key point to remember is that a population growth rate exceeding 2% per year is often seen as a signal for potential issues with resource management, healthcare, education, and employment. Countries with such high growth rates may need to implement specific policies to manage the rapid population increase.

## Correct Answer: C. 2%