State one use of radio-isotopes.

Explanation

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

1. **Understanding Radio-Isotopes**: Radio-isotopes are isotopes of elements that are radioactive, meaning they decay over time and emit radiation.

2. **Identifying Uses**: Radio-isotopes have various applications across different fields such as medicine, industry, and energy.

3. **Stating One Use**: One significant use of radio-isotopes is as fuel in nuclear reactors. They provide the necessary energy through nuclear fission processes.

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