In 1563, in Florence' s Palazzo Vecchio, Giorgio Vasari built in front of an existing wall a new wall on which he painted a mural. Investigators recently discovered a gap between Vasari' s wall and the original, large enough to have preserved anything painted on the original. Historians believe that Leonardo da Vinci had painted, but left unfinished, a mural on the original wall; some historians had also believed that by 1563 the mural had been destroyed. However, it is known that in the late 1560s, when renovating another building, Santa MariaNovella, Vasari built a facade over its frescoes, and the frescoes were thereby preserved. Thus, Leonardo' s Palazzo Vecchio mural probably still exists behind Vasari' s wall. Which of the following is an assumption on which the argument depends? ['When Vasari preserved the frescoes of Santa Maria Novella he did so secretly.', 'Vasari probably would not have built the Palazzo Vecchio wall with a gap behind it except to preserve something behind the new wall.', 'Leonardo would probably have completed the Palazzo Vecchio mural if he had had the opportunity to do so.', 'Leonardo rarely if ever destroyed artworks that he left unfinished.']