Ahistorian is writing a book about the fall of the soviet union. he claims that the country collapsed because it spent too much of its money on its military rather than providing domestic services to its people. what is the best way to determine whether the historian’s interpretation is valid? a. cite the historian's book in another study of the soviet union. b. corroborate the interpretation with other secondary sources. c. find out whether the historian has written any other books. d. find primary sources related to the collapse of other countries.