Consider the following statements regarding cyanobacteria like Nostoc and Anabaena: 1. They can fix nitrogen in both free-living and symbiotic states. 2. They possess specialized, thick-walled cells called heterocysts which provide an anaerobic environment for the nitrogenase enzyme. 3. Heterocysts lack Photosystem II, preventing oxygen evolution during photosynthesis while maintaining Photosystem I to generate ATP. Which of the statements given above are correct?

Options:

• A: 1 and 2 only
• B: 2 and 3 only
• C: 1 and 3 only
• D: 1, 2 and 3

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