Methane Should Be Rapidly Destroyed in Titan's Atmosphere, Yet It

Methane should be rapidly destroyed in Titan's atmosphere, yet it is still present in appreciable amounts. How can this be?

A) on the surface of Titan live cows that are constantly producing methane gas
B) larger hydrocarbons like ethane and propane are being broken down by ultraviolet light into methane
C) methane may be continually evaporating from the surface and the interior
D) comet impacts are constantly supplying the atmosphere with fresh methane