In a population of an area, the difference in the natality rate and mortality rate is zero in a particular year, but the population density has decreased. Which of the following may be the reason for this decrease ?

Options

A. Number of immigrants `gt` Number of emigrants
B. Number of immigrants / Number of emigrants = 1
C. Number of immigrants `lt` Number of emigrants
D. Number of immigrants = Number of emigrants

Answer

To solve the question regarding the decrease in population density despite a zero difference between natality (birth rate) and mortality (death rate), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Terms**:
- **Natality Rate**: The number of births in a population.
- **Mortality Rate**: The number of deaths in a population.
- **Immigration**: The arrival of new individuals into a population.
- **Emigration**: The departure of individuals from a population.

2. **Analyze the Given Information**:
- The difference between natality and mortality is zero. This means that the number of births equals the number of deaths in that year. Therefore, these two factors do not contribute to a change in population size.

3. **Identify Other Factors Affecting Population Density**:
- Since natality and mortality are balanced, the only factors left that can influence the population density are immigration and emigration.

4. **Evaluate the Impact of Immigration and Emigration**:
- **Immigration** would increase the population size as new individuals join the population.
- **Emigration** would decrease the population size as individuals leave the population.

5. **Conclude the Reason for Decrease in Population Density**:
- Since the population density is decreasing while natality equals mortality, it implies that more individuals are leaving the population (emigration) than are entering it (immigration). Therefore, the number of emigrants must be greater than the number of immigrants.

### Final Answer:
The reason for the decrease in population density is that the number of emigrants is greater than the number of immigrants.