Which one is not needed ?

Well... I would say it is gravity that is not needed.

Explanation:

In the picture at the top it says "we need to breathe air and live in some type of space. If these requirements are lacking, life ceases to exist."

Thus, stating that air, space, energy, and water are needed for life to exist. That leaves out gravity. Gravity is not needed for life to exist because, for example, humans and other living organisms are already living in outer space without the need for gravity.

Hope that helps!! :-)

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.