Answer:
a. Winter.
Step-by-step explanation:
The Nazi's attacked during the Summer (a classic mistake, made by a number of notable people, such as Napoleon). They failed to account for a long campaign (also like Napoleon), and so did not issue Winter clothing.
While in the summer, Russia is not as cold (which led to the issuing of standard military coats, the winter is extremely harsh. Coupled with natural causes (heavy snowfall), Russian raids on supply lines, and a long siege of cities held by the Nazi soldiers, the Nazi's started to starve, freeze, and surrendered soon after.
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