Nitrogenase, the Enzyme That Catalyses Nitrogen Fixation, Is Inhibited Whenever

Nitrogenase, the enzyme that catalyses nitrogen fixation, is inhibited whenever free oxygen (O₂) reaches a critical concentration. Consequently, nitrogen fixation cannot occur in cells wherein photosynthesis produces free O₂. Consider the colonial aquatic cyanobacterium, Anabaena, whose heterocysts are described as having "...a thickened cell wall that restricts entry of O₂ produced by neighbouring cells. Intercellular connections allow heterocysts to transport fixed nitrogen to neighbouring cells in exchange for carbohydrates".
Given that the enzymes that catalyse nitrogen fixation are inhibited by oxygen, what mechanism might nitrogen-fixing prokaryotes use to protect these enzymes from oxygen?

A) couple the nitrogen fixation enzymes with photosystem II (the photosystem that splits water)
B) package the nitrogen fixation enzymes in membranes that are impermeable to all gases
C) live only in anaerobic environments
D) package the nitrogen fixation enzymes in membranes that are impermeable to nitrogen gas (N₂) .