…………. gas is the main constituent of biogas.

Explanation

**Step-by-Step Solution:**

1. **Understanding Biogas**: Biogas is a type of renewable energy source that is produced from the breakdown of organic matter, such as plant and animal waste, through a process known as anaerobic digestion.

2. **Components of Biogas**: Biogas is not a single gas but a mixture of several gases. The primary gases found in biogas include methane (CH₄), carbon dioxide (CO₂), hydrogen (H₂), and hydrogen sulfide (H₂S).

3. **Identifying the Main Constituent**: Among these gases, methane is the most significant component. It is the gas that is primarily responsible for the energy content of biogas.

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