The leukaemia is caused by:

Options

A. Ca 40
B. Sr 90
C. caesium
D. iodine

Explanation

**Step-by-Step Solution:**

1. **Understanding Leukemia**: Leukemia is a type of cancer that affects the blood and bone marrow. It is characterized by the overproduction of abnormal white blood cells.

2. **Identifying Causes**: Various factors can contribute to the development of leukemia, including genetic predispositions and environmental exposures.

3. **Analyzing the Options**:
- **Option 1: Calcium-40**: Calcium is essential for bone health and does not cause leukemia.
- **Option 2: Strontium-90 (Sr-90)**: This is a radioactive isotope that can damage DNA and is linked to increased cancer risk, including leukemia.
- **Option 3: Cesium**: While cesium can be harmful in large amounts, it is not specifically known to cause leukemia.
- **Option 4: Iodine**: Iodine is crucial for thyroid health but is not associated with leukemia.

4. **Conclusion**: Based on the analysis, Strontium-90 is identified as the cause of leukemia due to its ability to damage DNA and its association with increased leukemia cases in populations exposed to it.

5. **Final Answer**: The correct answer is **Option 2: Strontium-90**.

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