When Albert Einstein Wrote to President Franklin Roosevelt in 1939

When Albert Einstein wrote to President Franklin Roosevelt in 1939 advocating a U.S. program to create a nuclear weapon, he justified it as necessary to deter:

A) a nuclear attack by Hitler's Germany
B) a nuclear attack by Hirohito's Japan
C) urban fire-bombings by the Germans
D) urban fire-bombing by the Japanese
E) none of the above