Give reason for the following:
Hydrogen has been replaced by helium or a mixture of hydrogen and helium to fill weather observation balloons.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Properties of Hydrogen:**
- Hydrogen is a highly flammable gas. When it comes into contact with air, it can easily ignite and cause explosions.

2. **Risk of Leakage in Balloons:**
- Weather observation balloons are often used at high altitudes, and there is a possibility of small leaks occurring in these balloons.

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