[A]: Drosophila is commonly used in the study of genetics.
[R]: Because it is very small and its life cycle is short.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**: The assertion states that "Drosophila is commonly used in the study of genetics." This is true because Drosophila melanogaster, commonly known as the fruit fly, has been a key model organism in genetic research.

2. **Understanding the Reason (R)**: The reason provided is "Because it is very small and its life cycle is short." This statement is also true. Drosophila is indeed small in size and has a short life cycle, typically around 10-14 days, which makes it convenient for genetic studies.

3. **Evaluating the Relationship Between A and R**: Now we need to determine if the reason (R) correctly explains the assertion (A). The small size and short life cycle of Drosophila make it easier and quicker to conduct genetic experiments, allowing researchers to observe multiple generations in a relatively short time.

4. **Conclusion**: Since both the assertion (A) and the reason (R) are true, and R provides a valid explanation for A, we conclude that both statements are correct and R is the correct explanation of A.

### Final Answer:
Both A and R are true, and R is the correct explanation of A.

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