Which of the following would a historian use to get a firsthand account of a past event

How did kennan’s long telegram shape the united states interaction with communism throughout the world? a. the united states sought to restrict the expansion of communism and soviet influence. b. the united states conquered communism by economically oppressing third-world nations. c. the united states created the european union to maintain control of its post-wwii influence. d. the united states sought to extend its influence by militarily forcing democracy on the world.