Answer:
Because Germany invaded the Soviet Union
Explanation:
After Germamy invaded the Societ Union and broke the treaty they teamed up with the allies.
well if the study was for regions then <em><u>D</u></em> would make the most sense
explanation: A would be wrong due to the fact it has nothing to do with the differences between regions
B would be wrong due to the fact its about a method not a ruler
c would be wrong due to the fact that it just doesn't meet your region requirement
d mentions regions & monarchy hence forth being your answer