The slow process by which the dead plants buried deep under the earth have become coal is called ______.

Explanation

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

1. **Understanding the Process**: The question asks about the slow process that transforms dead plants into coal. This process involves the decomposition of organic matter over a long period.

2. **Identifying the Process**: The specific term used to describe this transformation is crucial. The process is known as "carbonization."

3. **Defining Carbonization**: Carbonization is the process where organic materials, such as dead plants, are subjected to heat in the absence of air (destructive distillation). This leads to the conversion of these materials into coal.

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