A primary source is a historical record created by an individual who existed in the ancient eras.
The historical investigation that had the broadest spatial frame was the discovery of trade routes between Asian and European nations.
<h3>What is an investigation?</h3>
An investigation is a process of critically examining and identifying something related to any subject matter.
The sources formed by the ancient historians or individuals in their respective eras are defined as primary sources. They can be manuscripts, artistic creations, books, novels, etc.
The identification of trade routes through the silk road among the nations of Europe and Asia was one of the investigations of history having the largest frames.
Therefore, the record created by past individuals is the primary source, and the trade routes between Europe and Asia were the longest investigation.
Learn more about the primary sources in the related link:
The Second Agricultural Revolution's increased agricultural yields allowed a shift away from subsistence farming, leading to a decrease in subsistence farming rather than an increase.
