Read this and answer the question. If the question is unanswerable, say "unanswerable".

With Elizabeth's accession, it seemed probable that the royal house would bear her husband's name, becoming the House of Mountbatten, in line with the custom of a wife taking her husband's surname on marriage. The British Prime Minister, Winston Churchill, and Elizabeth's grandmother, Queen Mary, favoured the retention of the House of Windsor, and so on 9 April 1952 Elizabeth issued a declaration that Windsor would continue to be the name of the royal house. The Duke complained, "I am the only man in the country not allowed to give his name to his own children." In 1960, after the death of Queen Mary in 1953 and the resignation of Churchill in 1955, the surname Mountbatten-Windsor was adopted for Philip and Elizabeth's male-line descendants who do not carry royal titles.

What was the name of Elizabeth and Philip's last child?
unanswerable