How does this document/picture explain the reasons for the decline of the roman empire?

The decline of the Roman Empire was a process in which its huge territory was divided into several successor polities.

Explanation:

The Roman Empire lost the power that had allowed it to control the Western provinces. Thus, Barbarian's pressure over the Roman culture also contributed in an important way to its collapse.

Climate change was another element that is considered influential in these factors. During the Barbarian's invasion, the armed forces of the Western Empire became ineffective and the central rule was never consolidated.  

alaska is the answer to this

a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.