Write four characteristics of food chain.

Explanation

### Step-by-Step Solution:

1. **Definition of Food Chain**: A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. It typically starts with producers (like plants) and ends with top consumers (like carnivores).

2. **Characteristic 1 - Unidirectional Flow of Energy**: In a food chain, energy flows in one direction only, from producers to consumers. This means that energy is transferred from one trophic level to the next without reversing back to the previous level.

3. **Characteristic 2 - Trophic Levels**: A food chain usually consists of four to six trophic levels. These levels include producers, primary consumers, secondary consumers, and tertiary consumers, each representing a step in the energy transfer process.

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