In the study of history the available documents for studying a certain period is unchanging over time.
Option: B
Explanation:
History is the study of events and phenomena over space and time. Space means where the particular incident is taking place. Time means the period of occurrence of particular event. A certain period suppose the period of sultanate or mughal empire requires some documents, records to be studied in detail.
And these available documents, records can never be changed over time. History is study of things over changing space and time. Archaeological evidences also keep some valid documents for studying historical events.
