What Caused Early Astronomers to Believe That Our Galaxy Is

What caused early astronomers to believe that our galaxy is only about 6,000 light-years across?

A) Telescopes were not powerful enough to observe stars farther away.
B) Interstellar dust blocked visible light from stars farther away.
C) Stars farther away could not be resolved as individual objects.
D) Astronomers miscalculated the distances to stars, believing that the stars were 50 times closer than they actually were.
E) Astronomers assumed all red stars were faint main-sequence stars, and they confused them with the more luminous red giants.