Who won a 1939 Oscar for playing Mammy in "Gone with the Wind"?

In 1939, the warning "It may become possible to set up a nuclear chain reaction in a large mass of uranium" reached FDR… In the 1939 film "The Wizard of Oz," Dorothy says "I've a feeling we're not in Kansas anymore" to whom? Which 1939 movie includes the line "Pay no attention to that man behind the curtain"? In which 1939 western does John Wayne's character say, "My friends just call me Ringo. Nickname I had as a kid"? In "The Wizard of Oz" (1939), which actor played "that man behind the curtain"? "The Kingdom of Swing" (1939) was the autobiography of which bandleader? In which 1939 film does Laurence Olivier's Heathcliff say, "The moors and I will never change. Don't you, Cathy"? In 1939, the NAACP's Spingarn Medal went to which contralto, famous for both spirituals and opera?