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".
Think about this description of the colonial aquatic cyanobacterium, Anabaena. Which of the following questions below is important for understanding how nitrogen (N₂) enters heterocysts, and how oxygen (O₂) is kept out of heterocysts?

A) If carbohydrates can enter the heterocysts from neighbouring cells via the "intercellular connections", how is it that O₂ doesn't also enter via this route?
B) If the cell walls of Anabaena photosynthetic cells are permeable to O₂ and carbon dioxide (CO₂) , are they also permeable to N₂?
C) If the nuclei of the photosynthetic cells contain the genes that code for nitrogen fixation, how can these cells fail to perform nitrogen fixation?
D) If the nuclei of the heterocysts contain the genes that code for photosynthesis, how can these cells fail to perform photosynthesis?