How Do Some Cyanobacteria Fix Nitrogen While Growing Aerobically

How do some cyanobacteria fix nitrogen while growing aerobically?

A) they don't because nitrogenase is destroyed by oxygen
B) by using special cells called heterocysts that protect the enzyme
C) by using cysts that protect the enzyme
D) they just do it, no special structures or mechanisms
E) they live symbiotically with other organisms that protect the enzyme