On the oregon trail when crossing rivers the travelers had to be careful to avoid what that starts with an r?

Ahistorian is writing a book about british prime minister winston churchill. she claims that churchill was one of the earliest british leaders to realize that nazi germany was a serious threat to british security what is the best way to determine whether the historian's interpretation is valid? o a. review primary sources to see if churchill ever recorded his opinions of nazi germany. o b. find out whether the historian has written any other books about british leaders. c. analyze secondary sources summarizing the effects of nazi germany on great britain. o d. cite the historian's book in another study about important 20th- century world leaders.