How Is Nitrogen Thought to Have Become the Dominant Gas

How is nitrogen thought to have become the dominant gas in the atmosphere of the satellite Titan?

A) from the breakup of ammonia into nitrogen and hydrogen, with the hydrogen being lost to space
B) from microbes in the early atmosphere of Titan (when it was closer to the Sun)
C) from having a presence since primordial times, when Titan first formed
D) from outgassing through volcanic vents