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The build up of lactic acid will need oxygen to break it up. Lactic acid is why muscles feel tired after excess exercise
Answer:
These are things that won't be going away. Natural gasses and fossil fuels will likely run out someday or become much harder to find. When you use these sources of energy, they won't be going away and therefore will be sustainable because we can always continue using them.
Does that make sense?
Explanation:
A im guessing? soz if i get it wrong.