Answer:
I believe the answer is the Columbian Exchange.
Explanation:
I think the 3rd answer is correct
<span>the correct response would be that the historian is using "research skills", since he must find out the exact order of events. </span>
European Emigration to the U.S. 1851 - 1860
Although the Irish potato blight receded in 1850, the effects of the famine continued to spur Irish emigration into the 20th century. Still facing poverty and disease, the Irish set out for America where they reunited with relatives who had fled at the height of the famine.