Question: Who was the ruler ousted by the 30th Prime Minister of Cambodia after Khmer Rouge's victory over the Lon Nol government?

Context:
Khmer Rouge
The Khmer Rouge ( , ] , "Red Khmers"; Khmer: ខ្មែរក្រហម "Khmer Kror-Horm") was the name given to the followers of the Communist Party of Kampuchea in Cambodia.
 It was formed in 1968 as an offshoot of the Vietnam People's Army from North Vietnam, and allied with North Vietnam, the Viet Cong, and the Pathet Lao during the Vietnam War against the anti-communist forces from 1968 to 1975.
 The Khmer Rouge emerged victorious in the Cambodian Civil War, overthrew the military dictatorship of the Khmer Republic and installed their own government Democratic Kampuchea in 1975, led by Pol Pot, Nuon Chea, Ieng Sary, Son Sen, and Khieu Samphan.
 This was followed by the Cambodian genocide which occurred from 1975 until 1979, when the Khmer Rouge was finally removed from power by Vietnam in the Cambodian–Vietnamese War.
 The military power of the Khmer Rouge organization was not broken however, and a long era of guerrilla warfare involving large swaths of the Cambodian countryside began.
 Many different military factions were involved in the guerrilla war era and it ended around 1994.
Democratic Kampuchea
Kampuchea (Khmer: កម្ពុជា, "Kâmpŭchéa"), officially known as Democratic Kampuchea (Khmer: កម្ពុជាប្រជាធិបតេយ្យ, "Kâmpŭchéa Prâcheathippadey") (DK) from 5 January 1976, was the name of the Khmer Rouge (KR)-controlled state that, between 1975 and 1979, existed in present-day Cambodia.
 It was founded when the Khmer Rouge forces defeated the Khmer Republic of Lon Nol in 1975.
 During its rule between 1975 and 1979, the state and its ruling Khmer Rouge regime was responsible for the deaths of millions of Cambodians through forced labour and genocide.
 After losing control of most of Cambodian territory to Vietnamese occupation, it survived as a rump state supported by China.
 In June 1982, the Khmer Rouge formed the Coalition Government of Democratic Kampuchea with two non-communist guerilla factions, which retained international recognition.
 The state was renamed Cambodia in 1990 in the run up to the UN-sponsored Paris Peace Agreement conference of 1991.
Pol Pot
Pol Pot ( , ; Khmer: ប៉ុល ពត ; 19 May 1925 – 15 April 1998, born Saloth Sar; Khmer: សាឡុត ស ) was a Cambodian politician and revolutionary who led the Khmer Rouge from 1963 until 1997.
 From 1963 to 1981, he served as the General Secretary of the Communist Party of Kampuchea.
 As such, he became the leader of Cambodia on 17 April 1975, when his forces captured Phnom Penh.
 From 1976 to 1979, he also served as the Prime Minister of Democratic Kampuchea (30th Prime Minister of Cambodia).
Khun Srun
Khun Srun / ឃុន ស្រ៊ុន (1945–1978) was an important Cambodian writer.
 He was born in Char village, Roveang commune, Samrong district, Takéo province, into a poor Chinese Cambodian family.
 When he was eight, his father, Khun Kim Chheng, a Chinese man who had fled Communism, died, and he and his six siblings were raised by his mother, Chi Eng, a small shopkeeper and a devout Buddhist.
 He began his schooling during the country's first years of independence, when the doors to higher education and professionalization were inching open to all Cambodians, regardless of their social and economic class.
 A brilliant student, he studied Khmer literature and psychology at the university in Phnom Penh, becoming widely read in the sciences, mathematics, and European literature.
 Amid the turmoil of the 1960s, he worked as a professor of mathematics and a journalist while writing fiction and poetry.
 He also worked as a member of the textbook editorial committee at the Ministry of Education.
 In less than four years, he published three collections of poems, short tales, and philosophical anecdotes; two collections of autobiographical short stories, "The Last Residence" and "The Accused"; and a final volume of poems, "For a Woman".
 He was influenced by both existentialism and Cambodian Buddhism.
 In 1971, he was imprisoned during 7 months by the right-wing Lon Nol government for refusing to collaborate, but still refused to align himself with the extreme left.
 In 1973, after being imprisoned for a second time, he finally joined the communist guerrillas.
 He was only 28, and his life as a writer was finished.
 After the Khmer Rouge took power, in 1975, Khun Srun (aka Phoeun - ភឿន) was assigned work as a railway engineer).
 On the 20th of December 1978, he, his wife and their two youngest children were victims of the last purges.
 They were arrested, transferred to Tuol Sleng prison and probably killed in Choeung Ek, few days before the end of Pol Pot's regime.
 Only Khun Srun's nine-year-old daughter, Khun Khem, survived, taken by the Khmer Rouge and forced to live among them in the forest on the Cambodian-Thai border.
Norodom Kantol
Prince Norodom Kantol (15 September 19201976) was the 22nd Prime Minister of Cambodia, serving from 1962 to 1966.
 He also served as foreign minister under the Sangkum government led by his cousin, Norodom Sihanouk.
 During the Khmer Republic regime of Lon Nol, he was imprisoned along with other members of the royal family.
 He disappeared mysteriously in 1976, presumably killed by the Khmer Rouge.
 Kantol was a leading adviser of Norodom Sihanouk.
MONATIO
MONATIO, short for Mouvement National (French for "National Movement") was a short-lived, supposedly nationalist, political faction in Cambodia.
 The exact nature of the group is still obscure.
 On April 17, 1975, as the revolutionary forces came closer to the capital, this group took out a motor-cavalcade on the streets of Phnom Penh welcoming the arrival of the Khmer Rouge guerrillas.
 According to François Ponchaud's eye-witness report, the group consisted of a handful of soldiers, dressed in black uniforms, accompanied by a number of students.
 He claims MONATIO was led by Hem Keth Dara (the whole group in turn manipulated by Lon Non, brother of deposed Lon Nol).
 The Khmer Rouge regime later claimed that MONATIO had been a CIA conspiracy against the revolutionary government.
Economic history of Cambodia
Cambodia was a farming area in the first and second millennia BC.
 States in the area engaged in trade in the Indian Ocean and exported rice surpluses.
 Complex irrigation systems were built in the 9th century.
 The French colonial period left the large feudal landholdings intact.
 Roads and a railway were built, and rubber, rice and corn grown.
 After independence Sihanouk pursued a policy of economic independence, securing aid and investment from a number of countries.
 Bombing and other effects of the war during the Vietnam War damaged rice production.
 Lon Nol had a policy of liberalising the economy.
 This was followed by the victory of the Khmer Rouge and the emptying of the cities.
 After the defeat of the Khmer Rouge, a Five Year Plan was adopted, aiming to improve agriculture, industry and distribution, with a slogan of "export and thrift".
 Today, Cambodia remains a largely agricultural economy and industrial development is slow.
Penn Nouth
Samdech Penn Nouth (Khmer: សម្តេច ប៉ែន នុត , ] ) (April 1, 1906 – May 18, 1985) was a Cambodian politician.
 He served in the French colonial administration, then took active part in Cambodian politics, was several times a Prime Minister of Cambodia (1953, 1954-1955, 1958, 1961) as part of the Sangkum regime of Prince Norodom Sihanouk.
 He was the first prime minister of an independent Cambodia.
 He was prime minister for the sixth time from 31 January 1968 to 14 August 1969.
 On 18 March 1970, when Norodom Sihanouk was deposed by Lon Nol, Penn Nouth joined the monarch in his exile and became prime minister in the GRUNK coalition.
Soth Polin
Soth Polin / សុទ្ធ ប៉ូលីន (born February 9, 1943, in the hamlet of Chroy Thmar, Kampong Siem District, Kampong Cham Province, Cambodia) is a famous Khmer writer.
 His maternal great-grandfather was the poet Nou Kan (who wrote "Teav-Ek", ទាវឯក, a version of Tum Teav, the masterpiece of Cambodian love poetry).
 He grew up speaking both French and Khmer.
 Throughout his youth, he immersed himself in the classical literature of Cambodia and, at the same time, the literature and the philosophy of the West.
 His first novel, "A Meaningless Life", published in 1965 (he was 22 years old), was strongly influenced by Nietzsche, Freud, Sartre and Buddhist philosophy.
 It was an enormous success.
 Numerous novels and short stories followed, among them "The Adventurer With No Goal", "A Bored Man", "We Die Only Once", and "Dead Heart".
 He also worked as a journalist in "Khmer Ekareach" (The Independent Khmer), the newspaper of his uncle, Sim Var, and in the late 1960s, he founded (with Sin Kim Suy) the newspaper and publishing house, "Nokor Thom" (នគរធំ / The Great Kingdom).
 He was a militant nationalist who was both anti-Sihanouk and anti-communist.
 Through his newspaper, he supported the pro-American government of General Lon Nol before finally distancing himself and suddenly taking refuge in France in 1974, after the assassination of his friend, Thach Chea, the Deputy Minister of Education.
 His father and two of his brothers died during the Khmer Rouge regime.
 He worked in Paris as a taxi driver and published his dark cult novel "The Anarchist", written in French.
 Later he and his two sons moved to the West Coast of the United States, where he now resides.
Cambodian general election, 1976
General elections were held in Democratic Kampuchea on 20 March 1976, following the Khmer Rouge's victory over the Lon Nol government in April 1975.
 A total of 515 candidates for the 250 seats were put forward by the Communist-dominated National United Front of Cambodia, 150 of whom were to be elected from among the peasantry, 50 from the industrial workers, and 50 from the Kampuchean Revolutionary Army.
 Following the election, the newly elected Assembly convened on 11 April, electing a new administration with General Secretary Pol Pot as Prime Minister and his interim predecessor Khieu Samphan as President of the State Presidium, ousting Prince of Cambodia Norodom Sihanouk.
 Voter turnout was reported to be 98.0%.

Answer: Prince of Cambodia Norodom Sihanouk