## Core Concept
The population growth rate is a measure of how quickly a population increases in size over a given period, usually expressed as a percentage. The doubling time of a population is the time it takes for the population to double in size, which can be calculated using the population growth rate.

## Why the Correct Answer is Right
The formula to calculate the doubling time is: Doubling Time = 70 / Growth Rate (%). Given a population growth rate of 1.2-1.5%, we can calculate the doubling time as follows: For 1.2%, Doubling Time = 70 / 1.2 = 58.33 years; For 1.5%, Doubling Time = 70 / 1.5 = 46.67 years. The closest value to these calculations is 50 years for a 1.4% growth rate (using 70/1.4 = 50), which falls within the range provided.

## Why Each Wrong Option is Incorrect
- **Option A:** 20 years. This would imply a growth rate of 3.5% (70/20 = 3.5), which is much higher than 1.2-1.5%.
- **Option B:** 30 years. This would imply a growth rate of 2.33% (70/30 = 2.33), still higher than the given range.
- **Option D:** 100 years. This would imply a growth rate of 0.7% (70/100 = 0.7), which is lower than the given range.

## Clinical Pearl / High-Yield Fact
A commonly used rule of thumb for calculating population doubling time is the "Rule of 70," which states that you can calculate the doubling time by dividing 70 by the annual growth rate. This method provides a quick and easy way to estimate doubling times for various growth rates.

**Correct Answer: C. 50 years**