Gravity is a contact force that acts between two seperate subject

Answer(a); // soil salinization and waterlogging;

It is useful for historians to write narratives so that history can be passed on from their point of view.  think about a story being told in third person, then think about it again being told in first person, by someone who actually experienced the event.

To relate to humans in a relational humanly way. 

Iwould choose c or d