Consider the four pairs of elements below. Which two pairs are most likely to form an ionic bond?
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Ca and Al
N and o
Mg and S
Na and Br

Not what goes around on earth but the universe too.

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.