This statement was contained in a letter from Nikita Khrushchev to President Kennedy in 1962: "An act of aggression propelling humankind into the abyss of a world nuclear-missile war." To what is Khrushchev referring in this statement?

a. the establishment of Soviet missile sites in Cuba

b. the building of the Berlin Wall separating East and West Berlin

c. the U.S. support for the Bay of Pigs invasion

d. the U.S. naval blockade around Cuba