Why does the number of mitochondria vary in different cells?

Because some cells are bigger and hungrier :D The number of mitochondria a cell contains depends on how much energy the cell needs to produce. Muscle cells for example, contain more number of mitochondria compared to kidney cells. In addition, the number of mitochondria in a cell is not a constant.It varies depending on the physical activity of an individual.

To give you an example, it has been observed that marathon runners have more number of mitochondria in their leg muscle cells than people with desk jobs.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.