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, we need to analyze the situation described and understand the implications of natality, mortality, immigration, and emigration on population density.

### Step-by-Step Solution:

1. **Understanding Natality and Mortality Rates**:
- Natality rate refers to the number of births in a population per 1000 individuals in a given time period (usually a year).
- Mortality rate refers to the number of deaths in a population per 1000 individuals in the same time period.
- In this case, the difference between the natality rate and mortality rate is zero, meaning that the number of births equals the number of deaths.

2. **Implication of Zero Difference**:
- Since the natality and mortality rates are equal, we can conclude that the population size should remain stable if there are no other factors affecting it.

3. **Understanding Population Density**:
- Population density is defined as the number of individuals per unit area.
- If the population size remains stable (as indicated by equal natality and mortality), the population density should also remain stable unless there are changes in immigration or emigration.

4. **Considering Immigration and Emigration**:
- Immigration refers to individuals moving into a population from another area.
- Emigration refers to individuals leaving a population to move to another area.
- If the population density has decreased despite equal natality and mortality rates, this suggests that more individuals are leaving the population than are entering.

5. **Analyzing the Options**:
- **Option A**: The number of immigrants is greater than the number of emigrants. This would lead to an increase in population density, so this option is incorrect.
- **Option B**: The number of immigrants equals the number of emigrants. This would mean no net change in population size, hence no change in density, making this option incorrect.
- **Option C**: The number of immigrants is less than the number of emigrants. This means that more individuals are leaving the population than are coming in, which would lead to a decrease in population size and consequently a decrease in population density. This option is correct.
- **Option D**: The number of immigrants equals the number of emigrants. This is the same as Option B and is incorrect for the same reasons.

6. **Conclusion**:
- The correct answer is **Option C**: The number of immigrants is less than the number of emigrants.