Combustion! CH4, which is methane, reacts with oxygen to form carbon dioxide. (For future reference, if a compound is reacted with ONLY oxygen it is sure to be a combustion reaction)
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The oxidation state of a substance is the electric charge it is exhibiting in a given state. This may be determined by looking at the oxidation states of accompanying atoms as well as the charge on the complete molecule.
In this case:
Molecular charge: 0
Oxidation state of oxygen: -2 (it is a group 6 element)
Thus,
S + 3 * -2 = 0
S = 6
Sulfur is exhibiting an oxidation state of +6.