Which of the following is NOT true of the genocide in Cambodia?

a. The Khmer Rouge, Cambodia's Communist guerrillas, executed an estimated million and a half people over four years.
b. Mao Zedong declared war on Cambodia and began intense bombing in urban areas.
c. Forces from both Communist North Vietnam and the United States invaded Cambodia.
d. Pol Pot sealed off the country from the rest of the world and forced everyone to become farmers.