The two correct answers that effectively combine the sentences using a subordinating conjunction are:
B. Although the International Astronomical Union determined that Pluto did not meet the criteria needed to be considered a planet, Pluto was downgraded to the status of a dwarf planet in 2006.
E. After the International Astronomical Union determined that Pluto did not meet the criteria needed to be considered a planet, Pluto was downgraded to the status of a dwarf planet in 2006.