The centrally-located region is not immediately affected by the threat of desertification.

The centrally-located rain forests are not directly affected by the threat of desertification.

Explanation:

Desertification is a type of degrading lands where area of land loses its water bodies, vegetation and wildlife. This is by overexploitation of soil by humans through release of chemicals from industries, global warming which is a major cause for climate change. These are the results of geo-political and socio-economic conditions. This has become the major issue in sub-Sahara region of Africa and the borders of Southern edge of Africa. The centrally located regions are not immediately affected.  

A,) cause a & b are equal & both gives 180°