is the perceived frequency of a sound wave.

The answer would be -pitch-

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The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.

The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.