In 1917,Harlow Shapley Plotted the Positions of Almost a Hundred

In 1917,Harlow Shapley plotted the positions of almost a hundred globular clusters.But the galactic plane is cloaked by gas and dust,and the best way to "see" through this is with long-wavelength radio waves.However,in 1917 radio astronomy did not exist.How did Shapley accomplish his task?

A) He could only see globular clusters that were relatively close to the Sun.
B) He concentrated on using the long-wavelength end of the visible spectrum: red. This worked well because globular clusters are old and are dominated by red stars.
C) The globular clusters are mostly in a halo up off the galactic plane, so the dust was not a great problem.
D) In order to form a globular cluster, a great deal of interstellar dust is used up. Thus, globular clusters create holes in the dust and are relatively easy to see.