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Answer:
C. They were experienced military personnel from the Navajo Nation.
Explanation:
The Navajo code talkers were recruited as military personnel's by United States during world war II. But, They were not experienced soldiers. Most of them had regular jobs before they joined the world war.
Their part was to create a secret message using ancient Navajo language to other allies during the war. If the enemy intervened their message in the middle of the road, the message wouldn't be compromised by the opposition.
It would be A or D depending.
A if you are talking about the 1910's-20's. The war lead to some rapid growth in the military and made the USA into the central hub of trade in west as many nations were indebted to them.
D as this was the first time the USA became the global power as they took control of Cuba, Guam and the Philippines in this time.