In a certain code language, if 'sky' is known as 'earth', 'earth' is…

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 Z   Y    X   W   V    U   T    S    R   Q   P    O   N

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THE LOGIC FOLLOWED HERE IS:

WORD                 IS CALLED

SKY                        EARTH

EARTH                    WATER

WATER                      AIR 

AIR                          LIGHT

LIGHT                      SOUND

SO HUMANS BREATH IN AIR

AND AIR IS CALLED LIGHT

SO LIGHT IS CORRECT ANSWER