For humans, lactic acid fermentation occurs when you exert yourself to the point that your muscles can no longer get oxygen fast enough to generate energy via aerobic respiration. When this happens, your muscles produce energy by the far less efficient process of lactic acid fermentation. This is also true in most other mammals In bacteria, however lactic acid fermentation can either be the normal method that a bacteria has for producing energy, or it can be a method used when there is no oxygen available. Which of these will be the case depends on the type of bacteria.
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The answer is Romeo's family and Juliet's family are enemies.
