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.