Which statement is true of an object in equilibrium? a. the sum of all the horizontal forces is nonzero. b. the sum of all the vertical forces is nonzero. c. the sum of all forces acting on the object is zero. d. at least one of the forces acting on the object is zero.

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.