__________ gas is useful for 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 is essential for the survival of these organisms as it allows them to produce their own food.

2. **Identifying the Requirements for Photosynthesis**: For photosynthesis to occur, several key components are needed:
- Sunlight: Provides the energy required for the process.
- Chlorophyll: A green pigment found in plants that captures light energy.
- Carbon Dioxide: A gas that plants take in from the atmosphere.
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