From 2003 to 2005, astronomers discovered several Kuiper Belt objects comparable in size to Pluto, leading them to reconsider what counts as a planet. Under the definition the International Astronomical Union adopted in 2006, a full planet must have cleared its orbital neighborhood; Pluto has not. ______ Pluto does not qualify as a full planet and is instead classified as a dwarf planet.
Which choice completes the text with the most logical transition?