D. All of the above
A. In the event of FDR's death, he was replaced by President Truman, who hated the communist system, and worked hard to keep communism from spreading.
B. As both were counted as the "world-powers", infrastructure, economies, etc. have to be strong in order to have a strong economic power, and in turn, be a strong global power. As the US suffered less damage (which includes small structural damages in mainland US due to fire balloons, attacks on Alaska, and the attack on Pearl Harbor), the US generally left the war unscathed from the war, which led to them being able to progress. However, the USSR suffered large amounts of damage to their infrastructure and population, and had to rebuild.
C. While this is not really a reason, because the top 2 counts, so I'm assuming this also occurred through the form of propaganda, in which the US had to portray the USSR as the enemy, and the pact between Stalin & Hitler in the beginning of the war was one of the reasons the US propaganda team jumped upon to use.
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