Name the source of `O_2` during photosynthesis.

Explanation

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This process also produces oxygen as a by-product.

2. **Key Components of Photosynthesis**: The main components required for photosynthesis are carbon dioxide (CO₂), water (H₂O), and sunlight. These components are essential for the plant to create its food.

3. **Reactions in Photosynthesis**: Photosynthesis consists of two main types of reactions:
- **Light Reactions**: These occur in the presence of sunlight and involve the conversion of solar energy into chemical energy.
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