Aparticle moves along the x-axis so that at time t > (or equal to) 0 its position is given by x(t)=2t^3 - 21t^2 + 72t-53. at what time t is the particle at rest?

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.