Titan's Thick Atmosphere Is Believed to Have Been Created When

Titan's thick atmosphere is believed to have been created when ultraviolet photons broke apart methane molecules, ultimately creating the observed smog-like conditions. This process, though, would likely remove all of the atmospheric methane in roughly 10 million years, yet we still see its presence today. What is the most likely cause?

A) Cometary impacts periodically bring new methane to Titan.
B) Ethane rains down out of the atmosphere, combines with surface rocks, and creates new methane.
C) Infrared photons give atmospheric molecules enough energy to recombine into methane.
D) Volcanoes on Titan periodically release new methane into the atmosphere.
E) Bacteria on Titan constantly replenish the methane in the atmosphere.