The Possible Presence of a Very Large Amount of Unseen

Multiple Choice

The possible presence of a very large amount of unseen ("dark") matter in the halo of our Galaxy is deduced from the

A) unexpected absence of luminous matter (stars,etc. ) beyond a certain distance.
B) rotation curve of our Galaxy,which indicates higher than expected orbital speeds in the outer regions of the Galaxy.
C) rotation curve of our Galaxy,which shows that orbital speeds in the outer parts of the Galaxy decrease in a way that follows Kepler's law.
D) unexpected high amount of interstellar absorption in certain directions.