An aerobic activity is defined as one that the heart rate and the body's need for oxygen.
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The term aerobic literally means "with oxygen", and refers to the use of oxygen by muscles for generating energy. An aerobic activity will increase the rate of heat so that it can pump more oxygenated blood because of increase in muscle activity, the need of oxygen of body also increases.  

Hence, an aerobic activity is defined as one that increases the heart rate and increases the body's need for oxygen.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

affects the water bodies: excessive use of fertilizers causes eutrophication. the fertilizers which are used on plants affect the quality of land on which it is used, further causing erosion of soil. the fertilizers used on plants contain harmful chemical fumes which affect the environment.