What Was Especially Noteworthy About the Sagittarius Dwarf Galaxy When

What was especially noteworthy about the Sagittarius Dwarf Galaxy when it was discovered among the small galaxies near the Milky Way?

A) it was so large in mass, astronomers could not understand how the Milky Way wasn't flying toward it
B) it had a supermassive black hole in the center much larger than the Milky Way's
C) it was rotating so quickly that it could not hold together for very long
D) it was on a collision course with the Milky Way and would be swallowed by it eventually
E) it was made entirely of dark matter and so could not be seen on any photographs