How do you know if the author is credible?

Will give brainliest what changes took place in the soviet union and eastern europe after ww2

In what year did vladimir lenin and the bolsheviks seize power in russia? 1885 1900 1917 1961

Which source would best a historian understand the effect of the great chicago fire on families? a. a biography of a chicago family displaced by the fire b. a letter from a grandmother in new york to her family in chicago c. a newspaper editorial about families who were displaced d. an oral history dictated by an adolescent who fled the fire