What does it mean to say “force of drag”

It is a mechanical force generated by a solid object moving through a surrounding fluid. In other words, you can also say that drag is a force acting opposite to the relative motion of any object moving through a surrounding fluid.

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.

Answer(b) both involve the use of levers;