What fraction of a given sample of radioactive nuclide remains after four half lives? ?

After each half life, the amount is halved.
After n half lives, the remaining fraction is
1/(2ⁿ)
After four half lives:
1/2⁴
= 1/16 of the original amount

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.