The atmosphere in a tank is too rich when it is _____.

A
incapable of supporting combustion because the hydrocarbon vapor content makes the atmosphere below the LFL (Lower Flammable Level)
B
capable of supporting combustion
C
in a noncombustible state which can be relied on to occur naturally on a regular basis
D
incapable of supporting combustion because the hydrocarbon vapor content makes the atmosphere above the UFL (Upper Flammable Limit)
E