Fossil fuels are ____, ______ and _____

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Fossil Fuels**:
- Fossil fuels are natural substances that are formed from the remains of ancient plants and animals. These organic materials have undergone a process of decomposition over millions of years under specific conditions.

2. **Formation Process**:
- The formation of fossil fuels occurs under the earth's crust where there is significant pressure and heat. This environment facilitates the transformation of the organic matter into usable energy sources.

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