What Observations Suggest the Mass of the Galaxy Goes Much

Multiple Choice

What observations suggest the mass of the Galaxy goes much farther out than its visible disc?

A) 21 cm maps of the spiral arms
B) the rotation curve of the outermost portions of the disc
C) the orbits of the open clusters in the disc
D) infrared observations of distant brown dwarfs
E) X-ray images of other galaxies' discs from Chandra