[A]: B-cells are responsible for antibody mediated immunity which directly recognize an antigen and give rise to antibody secreting plasma cells and memory cells.
[R]: Immunity can be induced by using vaccines for long lived active period.

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**:
- The assertion states that B-cells are responsible for antibody-mediated immunity. This means that B-cells can recognize specific antigens (foreign substances) and respond by producing antibodies.
- B-cells differentiate into plasma cells, which secrete antibodies, and memory cells, which help in long-term immunity.

2. **Understanding the Reason**:
- The reason provided states that immunity can be induced by using vaccines for a long-lived active period. Vaccines introduce antigens into the body, prompting the immune system to produce antibodies without causing the disease.
- This process leads to the formation of memory cells that provide long-lasting immunity.

3. **Evaluating the Truth of the Assertion and Reason**:
- Both the assertion and the reason are true. B-cells do play a crucial role in antibody-mediated immunity, and vaccines do help in inducing immunity.

4. **Determining the Relationship Between Assertion and Reason**:
- While both statements are true, the reason does not serve as a correct explanation for the assertion. The assertion focuses on the role of B-cells in immunity, while the reason discusses the role of vaccines in inducing immunity.

5. **Conclusion**:
- Since both the assertion and reason are true, but the reason does not explain the assertion, the correct answer is that both are true, but the reason is not the correct explanation of the assertion.

### Final Answer:
The correct option is: **Both assertion and reason are true, but reason is not the correct explanation of assertion.**

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