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

In the 1990s two icons by the Russian icon painter Sergei Fyodorov were hung in the abbey. On 6 September 1997 the funeral of Diana, Princess of Wales, was held at the Abbey. On 17 September 2010 Pope Benedict XVI became the first pope to set foot in the abbey.

Who was the last pope to set foot in the abbey?
unanswerable