________ is formed when charcoal is burnt in a limited supply of air.

Explanation

**Step-by-Step Solution:**

1. **Identify the Substance**: The question mentions "charcoal," which is primarily composed of carbon.

2. **Understand the Burning Process**: When a substance burns, it reacts with oxygen. In this case, charcoal (carbon) is being burnt in air, which contains oxygen.

3. **Consider the Supply of Air**: The question specifies that the burning occurs in a "limited supply of air." This is crucial because the amount of oxygen available will affect the products formed during combustion.

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