Name a gas used in photosynthesis.

Answer

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis is the process by which plants make their own food using sunlight, carbon dioxide, and water.

2. **Identifying the Gases Involved**: During photosynthesis, plants take in gases from the environment. The primary gas that is used in this process is carbon dioxide.

3. **Role of Carbon Dioxide**: Carbon dioxide is absorbed from the air through small openings in the leaves called stomata. This gas is essential for the production of glucose, which is the food for the plant.

4. **Conclusion**: Therefore, the gas used in photosynthesis is carbon dioxide.

**Final Answer**: Carbon Dioxide.

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