Is the natural resource that has played the largest role in the economic development of the persian gulf countries. -petroleum -silver -salt -gold

Your answer would be Petroleum. 
I hope this helps, Good luck! :) 

Petroleum is the answer

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.

Answerhighlands and forests;