A historian 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?

A Find out whether the historian has written any other books about British leaders

B. Review primary sources to see if Churchill ever recorded his opinions of Nazi Germany.

C. Cite the historian's book in another study about important 20th century World leaders D.

D. Analyze secondary sources summarizing the effects of Nazi Germany on Great Britain.​