What was the key issue in the breakdown of relations between China and the Soviet Union?

What was the key issue in the breakdown of relations between China and the Soviet Union?
 
  a. China believed that the Soviets had demonstrated socialist technological superiority and should to on the offensive.
  b. Khrushchev argued that China had not reached an elevated state of Communism according to Marxist ideology.
  c. Mao rebuked Khrushchev for his policy of de-Stalinization.
 d. Mao felt that Khrushchev had disgraced Communism after the Cuban Missile Crisis.
 e. Russia's increasing closeness with Castro and communist regimes in Latin America.