Main source of helium is

Options

A. Air
B. Redium
C. Monazite
D. Water

Explanation

**Step-by-Step Solution:**

1. **Understanding Helium Sources**: The question asks for the main source of helium. Helium is a noble gas that is found in trace amounts in the Earth's atmosphere and is also produced through certain geological processes.

2. **Identifying Helium in the Atmosphere**: Helium constitutes about 0.0005% of the Earth's atmosphere. While it is present in the air, this is not the primary source of helium.

3. **Exploring Geological Sources**: Helium is primarily produced through the radioactive decay of heavy elements in the Earth's crust. This process generates alpha particles, which are essentially helium nuclei.

4. **Monazite Sand**: Monazite is a reddish-brown phosphate mineral that contains rare earth metals such as thorium, lanthanum, and cerium. It is significant because it can release helium when subjected to radioactive decay processes.

5. **Radiation and Helium Production**: When monazite sand is exposed to radiation (such as X-rays), it can produce alpha particles (helium nuclei) as a byproduct of the decay of radioactive elements present in the mineral.

6. **Conclusion**: Based on the information, the main source of helium is monazite sand due to its content of radioactive elements that produce helium through decay.

**Final Answer**: The main source of helium is monazite sand.

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