because the x value is not repeated twice as it is in {(-8, -2), (7, -2),(-9,2), (0,0) {(5,8), (9,7),(-3,-8),(5,-5)} {(7,-8), (7, - 7), (8,4),(1, -3)}
Oxygen and methane can react chemically with each other, so we would not see them together unless there are active sources for both. On Earth, biology is responsible for essentially all the oxygen and the majority of the methane in our atmosphere.