The diagram shows a simple lipid!
what type of lipid is shown?

the correct answer is the nitrogen cycle.  

nitrogen cycle refers to an array of procedures by which nitrogen and its compounds are interchanged in the living organisms and in the environment, comprising decomposition and nitrogen fixation. there is a need for the plants to convert nitrogen into the usable form to be utilized by the living species. the cycling of the nitrogen takes place via four procedures, that is, decomposition, nitrogen fixation, denitrification, and nitrification.  

the answer is a. because the solar system is built around the sun and not the earth, and since there are only 8 planets half of that would be four.