What are the types of evidence the geographers and the historian use to explain importans topics in the fileds

They don't collect evidence from just one source but a variety of sources and look for similarities in accounts. They also work backwards from the topic; so first getting the eye witness/directly involved accounts then getting the second hand accounts etc. 

Since it's impossible to check everything all time (or in time when talking history) the thoroughness of the expert is important. The layers of expertise are also important as experts often build on the work of the previous experts before them.

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

Ba rock that formed 1,400 million years ago!