When a big fish eats a small fish which eats water fleas supported by phytoplankton, the water fleas are:

Options

A. Producer
B. Primary consumers
C. Secondary consumers
D. Top consumer in this top chain.

Answer

To answer the question regarding the classification of water fleas in the food chain, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components of the Food Chain**:
- The food chain starts with **phytoplankton**, which are the primary producers.
- The next level consists of **water fleas**, which consume phytoplankton.
- Following that, **small fish** eat the water fleas.
- Finally, a **big fish** consumes the small fish.

2. **Define the Role of Phytoplankton**:
- Phytoplankton are autotrophic organisms that perform photosynthesis to produce energy. They are categorized as **producers** in the ecosystem.

3. **Determine the Role of Water Fleas**:
- Water fleas feed on phytoplankton, which means they directly consume the producers. Hence, they are classified as **primary consumers**.

4. **Identify the Next Trophic Levels**:
- The small fish that eat the water fleas are classified as **secondary consumers** because they consume primary consumers.
- The big fish that eats the small fish are classified as **tertiary consumers**.

5. **Conclusion**:
- Therefore, in the context of the food chain described, water fleas are classified as **primary consumers**.

### Final Answer:
Water fleas are **primary consumers**.