The —- is our principal source of energy.

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for the principal source of energy.

2. **Recall Basic Knowledge**: Think about the various sources of energy available on Earth. Consider natural sources like fossil fuels, wind, water, and the sun.

3. **Determine the Most Important Source**: Among these sources, the sun is the most significant because it provides energy that supports life and various energy forms.

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