The scientists believed that since the first product was `C_(3)` acid, the primary `CO_(2)` acceptor would be -

Options

A. `C_(2)` - compound
B. `C_(3)` - compound
C. `C_(5)` - compound
D. `C_(6)` - compound

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the primary CO2 acceptor in the context of photosynthesis, specifically when the first product formed is a C3 acid.

2. **Identifying the C3 Acid**: In the Calvin cycle of photosynthesis, the first stable product formed after the fixation of CO2 is 3-phosphoglycerate (3-PGA), which is a 3-carbon compound (C3 acid).

3. **Determining the CO2 Acceptor**: The scientists initially hypothesized that if the first product is a C3 acid, then the primary CO2 acceptor would likely be a C2 compound (a 2-carbon compound). This is based on the idea that a 2-carbon compound could combine with a carbon from CO2 to produce a 3-carbon compound.

4. **Correct Identification of the CO2 Acceptor**: However, further research revealed that the actual primary CO2 acceptor in the Calvin cycle is Ribulose 1,5-bisphosphate (RUBP), which is a 5-carbon compound. When CO2 is fixed to RUBP, it forms a 6-carbon intermediate that quickly breaks down into two molecules of 3-PGA (the C3 acid).

5. **Conclusion**: Therefore, the primary CO2 acceptor in the Calvin cycle is RUBP (Ribulose 1,5-bisphosphate), which is a 5-carbon compound.

### Final Answer:
The primary CO2 acceptor is Ribulose 1,5-bisphosphate (RUBP), a 5-carbon compound.

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