$ If natality is greater than mortality, it leads to population explosion.
! The scientific study of various aspects of human population is called demography.

Options

A. If both As and R are correct and R is correct explanation of As
B. If both As and R are correct but R is not correct explanation of As
C. If As is correct, R is wrong
D. If both As and R are wrong.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that if natality (birth rate) is greater than mortality (death rate), it leads to a population explosion.
- This means that when more individuals are born than die, the population will increase rapidly.

2. **Understanding the Reason**:
- The reason given is that the scientific study of various aspects of human population is called demography.
- Demography encompasses the study of birth rates, death rates, and population dynamics.

3. **Evaluating the Assertion**:
- The assertion is correct because a higher birth rate than death rate indeed results in population growth, which can lead to a population explosion.

4. **Evaluating the Reason**:
- The reason is also correct as it accurately defines demography. However, it does not directly explain the assertion about population explosion.

5. **Determining the Relationship**:
- While both statements are correct, the reason does not explain why an increase in natality over mortality leads to population explosion. The reason focuses on the study of population rather than the dynamics of natality and mortality.

6. **Selecting the Correct Option**:
- Based on the evaluations, the correct option is:
- **Option B**: Both assertion and reason are correct, but R (reason) is not the correct explanation of A (assertion).