The answer for this question is C. The Domino Theory. This theory states that if a country came under communist rule, its neighboring countries and regions would follow as well. This theory is what the US used as a reason to intervene in the different affairs of various countries to stop or prevent communism from spreading.
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everything can be found in the picture
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it is like a secret code that you can use to talk to people when you don't want an enemy to find out your tactics.
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B. population of the region