A historian who compares world events during the classical era to world events during the modern era is organizing history by?

Period

Explanation:

In history, period defines a specific time frame when relevant historical events were occurring rather than focusing on one single event.

One characteristic of historical organization using period is the use of the word 'Era' to distinguish one historical period to another. We can see this in the example above.

Classical Era happened between 8th century BC and the 6th century AD

Modern Era happened between the year of   1500 to around 1800

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.

"a government in which power is divided between state and national levels". federalism is a system of government in which entities such and state and province, share power with the national government.