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Which of the following cannot fix nitrogen?

1. Nostoc
2. Azotobacter
3. Spirogyra
4. Anabaena

Correct answer: Spirogyra

Solution

Spirogyra is a filamentous green alga and does not fix atmospheric nitrogen. Nostoc and Anabaena are cyanobacteria with nitrogen-fixing cells, and Azotobacter is a free-living nitrogen-fixing bacterium.

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