Which material is a material is mixture

A heterogeneous mixture is a mixture of two or more chemical substances (elements or compounds). Examples are: mixtures of sand and water or sand and iron filings, a conglomerate rock, water and oil, a portion salad, trail mix, and concrete (not cement).

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A mixture is two substances mixed together but not bonded together. Methane is CH4, so again it is bonded together so again it is not a mixture. Magnesium is just an element so there is nothing mixed in it since it is a pure substance. So the answer is Air since it is a mixture of gases.