Assertion: The first product of `CO_(2)` fixation in `C_(3)` pathway is OA A.
Reason: The first product of `CO_(2)` fixation in `C_(4)` pathway is PGA.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the first product of CO₂ fixation in the C₃ pathway is OAA (oxaloacetic acid).
- **Evaluation**: This is incorrect. The first product of CO₂ fixation in the C₃ pathway is actually 3-phosphoglyceric acid (PGA), which is a three-carbon compound.

2. **Understanding the Reason**:
- The reason states that the first product of CO₂ fixation in the C₄ pathway is PGA (3-phosphoglyceric acid).
- **Evaluation**: This is also incorrect. In the C₄ pathway, the first product of CO₂ fixation is indeed OAA (oxaloacetic acid), which is a four-carbon compound.

3. **Conclusion**:
- Since both the assertion and the reason are incorrect, we conclude that the statement provided in the question is false.

### Final Answer:
Both the assertion and the reason are false.

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