In order to fully understand an author's message, the reader must first A. identify the climax and the actions leading up to it
B. identify the characters and their motivation
C. identify the tone of the introduction
D. identify the topic and its impact on human kind

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.