Respuesta :
"effective secondary source document" would be the best option in terms of a chart developed 20 years after a historical event, but this is not always true.
The answer is: effective secondary source document"
Within 20 years, historians would have enough time to gather all facts and data from all sides that involved in historical event. All of these facts and data is useful for them to decipher how the historical events were unfolded.
This make the chart an effective source to find for information regarding the event. But, since it does not made by people who actually involved in the events, it could only be considered as a secondary source.
