In 1960 which of the following contributed most directly to Soviet leader Nikita Khrushchev's cancellation of a scheduled summit meeting with President Dwight D. Eisenhower?:
 A. the rise to power of Fidel Castro in Cuba.
 B. the failure, at the Bay of Pigs, of a U. S.-sponsored attempt to oust Fidel Castro.
 C. the downing of an American U-2 spy plane over the Soviet Union.
 D. the sending of U. S. troops to Lebanon.
 E. the success of the Soviet space program in launching the Sputnik satellite.