Blue-green algae are

Options

A. Eubacteria
B. Cyanobacteria
C. Actinomycetes
D. Archaebacteria.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about blue-green algae and their classification.

2. **Identifying Blue-Green Algae**: Blue-green algae are commonly known as cyanobacteria. This is a crucial point as it directly relates to the classification of these organisms.

3. **Classification of Cyanobacteria**: Cyanobacteria fall under the category of Eubacteria. Eubacteria are true bacteria and include various types, including cyanobacteria.

4. **Characteristics of Cyanobacteria**:
- **Pigments**: Cyanobacteria contain two main pigments:
- Phycocyanin (which gives them a blue color)
- Chlorophyll (which gives them a green color)
- **Cell Structure**: They are unicellular and prokaryotic, meaning they lack a true nucleus.
- **Nutritional Mode**: Cyanobacteria are autotrophic, which means they can produce their own food through photosynthesis.

5. **Conclusion**: Based on the classification and characteristics discussed, blue-green algae are classified as cyanobacteria.

### Final Answer:
Blue-green algae are classified as cyanobacteria.

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