State two necessary conditions for hearing a distance echo.

Explanation

To answer the question regarding the necessary conditions for hearing a distant echo, we can break it down into two main points:

### Step-by-Step Solution:

1. **Minimum Distance Requirement**:
- The first necessary condition for hearing an echo is that the distance from the source of sound to the reflecting object (like a wall or a mountain) must be at least 17.2 meters. This distance ensures that the sound wave has enough time to travel to the object and back to the listener.

2. **Time Interval Requirement**:
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