What Physical Process Has Allowed Astronomers to Measure the Overall

What physical process has allowed astronomers to measure the overall number and distribution of brown dwarfs in the Milky Way Galaxy?

A) The gravitational fields of brown dwarfs can bend the light from background stars.
B) Brown dwarfs emit copious amounts of infrared radiation, which can penetrate the interstellar medium.
C) Molecules in the atmospheres of brown dwarfs emit radio waves that can be detected from Earth.
D) Brown dwarfs occasionally eclipse (block the light from) more distant stars.