Passage: Mahathir had been politically active since the end of the Japanese occupation of Malaya, when he joined protests against the granting of citizenship to non-Malays under the short-lived Malayan Union. He later argued for affirmative action for Malays at medical college. While at college he contributed to The Straits Times under the pseudonym "C.H.E. Det", and a student journal, in which he fiercely promoted Malay rights, such as restoring Malay as an official language. While practising as a doctor in Alor Setar, Mahathir became active in UMNO; by the time of the first general election for the independent state of Malaya in 1959, he was the chairman of the party in Kedah. Despite his prominence in UMNO, Mahathir was not a candidate in the 1959 election, ruling himself out following a disagreement with then Prime Minister Tunku Abdul Rahman. The relationship between the two Kedahans had been strained since Mahathir had criticised Abdul Rahman's agreement to the retention of British and Commonwealth forces in Malaya after independence. Now Abdul Rahman opposed Mahathir's plans to introduce minimum educational qualifications for UMNO candidates. For Mahathir this was a significant enough slight to delay his entry into national politics in protest. The delay did not last for long. In the following general election in 1964, he was elected as the federal parliamentarian for the Alor Setar-based seat of Kota Setar Selatan.Elected to parliament in a volatile political period, Mahathir, as a government backbencher, launched himself into the main conflict of the day: the future of Singapore, with its large and economically powerful ethnic Chinese population, as a state of Malaysia. He vociferously attacked Singapore's dominant People's Action Party for being "pro-Chinese" and "anti-Malay" and called its leader, Lee Kuan Yew, "arrogant". Singapore was expelled from Malaysia in Mahathir's first full year in parliament. However, despite Mahathir's prominence as a backbencher, he lost his seat in the 1969 election, defeated...

Question: Who was mainly responsible for the resignation of prime minister according to Diane?
Mahathir