Which feature of some species of blue- green algae helps promote them as bio-fertilizers ?

1. Bio-fertilizers enhance soil fertility using microorganisms. 2. Certain species of blue-green algae (cyanobacteria) possess specialized cells called heterocysts. 3. These algae have the mechanism (nitrogenase enzyme system within heterocysts) to fix atmospheric nitrogen ($N_2$) directly into ammonia ($NH_3$) and other nitrogenous compounds. 4. These nitrogenous compounds are in a form that crop plants can absorb readily for their growth, thus acting as a bio-fertilizer. 5. Option A is incorrect as they fix nitrogen, not methane, into ammonia. 6. Option B is incorrect; the algae perform the conversion themselves, not induce plant enzymes. 7. Option D is incorrect; they provide new nitrogen, not just enhance uptake of existing soil nitrates.