Elizabeth II:

According to Paul Martin, Sr., by the end of the 1970s the Queen was worried that the Crown "had little meaning for" Pierre Trudeau, the Canadian Prime Minister. Tony Benn said that the Queen found Trudeau "rather disappointing". Trudeau's supposed republicanism seemed to be confirmed by his antics, such as sliding down banisters at Buckingham Palace and pirouetting behind the Queen's back in 1977, and the removal of various Canadian royal symbols during his term of office. In 1980, Canadian politicians sent to London to discuss the patriation of the Canadian constitution found the Queen "better informed ... than any of the British politicians or bureaucrats". She was particularly interested after the failure of Bill C-60, which would have affected her role as head of state. Patriation removed the role of the British parliament from the Canadian constitution, but the monarchy was retained. Trudeau said in his memoirs that the Queen favoured his attempt to reform the constitution and that he was impressed by "the grace she displayed in public" and "the wisdom she showed in private".

Please answer a question about this article. If the question is unanswerable, say "unanswerable". In what year did Pierre Trudeau become the Canadian prime minister?
unanswerable