## Core Concept
The crude birth rate (CBR) is a demographic measure that calculates the number of live births per 1,000 people in a population over a specific period, usually a year. It is an essential indicator of population growth and fertility. The CBR is calculated using a straightforward formula.

## Why the Correct Answer is Right
The correct answer, **B. Mid-year population**, is used as the denominator because it provides a more accurate representation of the population at risk during the period in which the births occurred. The mid-year population is the average population during the year, which helps to smooth out changes in population size due to births, deaths, and migration. Using the mid-year population as the denominator allows for a more precise calculation of the rate.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because the total population at the beginning of the year does not accurately represent the population at risk throughout the year due to changes in population size.
* **Option C:** This option is incorrect because the total population at the end of the year also does not accurately represent the population at risk throughout the entire year.
* **Option D:** This option is incorrect because the number of women of reproductive age is used in calculating the general fertility rate, not the crude birth rate.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the crude birth rate does not take into account the age and sex distribution of the population, which is why it's termed "crude." For more specific fertility measures, like the general fertility rate, the denominator would indeed be the number of women of reproductive age (usually 15-44 or 15-49 years).

## Correct Answer: B. Mid-year population