......................exists in the form of buckyball and nanotubes.

Answer

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for a substance that exists in the forms of "buckyball" and "nanotubes".

2. **Understand the Terms**:
- **Buckyball**: This refers to a specific structure of carbon known as fullerene, which resembles a soccer ball or a geodesic dome.
- **Nanotubes**: These are cylindrical structures made of carbon atoms arranged in a hexagonal pattern, which are derived from fullerenes.

3. **Determine the Answer**: The substance that exists in both forms is **fullerene**.

4. **Explain Fullerene**: Fullerene is an allotropic form of carbon. Allotropes are different forms of the same element, where the atoms are bonded together in different ways.

5. **Relate to Properties**: Fullerenes, including buckyballs, have unique properties such as high strength, electrical conductivity, and thermal conductivity, making them useful in various applications, including the creation of carbon nanotubes.

6. **Final Answer**: Therefore, the answer to the question is **fullerene**.

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