I think, they rely on primary sources because these sources are original and more trustworthy
The Great Migration, sometimes known as the Great Northward Migration or the Black Migration, was the movement of 6 million African Americans out of the rural Southern United States to the urban Northeast, Midwest, and West that occurred between 1916 and 1970.
The answer is strong governments in towns and villages, and a relatively weak government.
True I think look in your textbook
