The threat of history refers to . some subjects who have had history classes and others who have not the realization that some subjects are much older than others events external to the experiment that will impact the results the fact that only current, relevant subjects need be studied

 External event can impact research through being present outside

Explanation:

According to question, threat of history is the factor that can impact outcome of the research in damaging manner.External event and factors taking place outside the historical research can influence result and modify it through manipulation of involved inputs and techniques.

Other options are incorrect because subject being older than other subject, studying of only current subject ,some subject are history related and some without realization do not create threat of history.

answer; overt behavior;