How Did Astronomers Determine That Elliptical Galaxies Are Composed Mostly

How did astronomers determine that elliptical galaxies are composed mostly of dark matter?

A) They measured the velocities of stars in the inner regions of the galaxies.
B) They measured the rotation rates of gas as a function of distance from the centers of the galaxies.
C) They measured the amount of gravitational pull elliptical galaxies have on companion galaxies.
D) They assumed that the proportion of dark matter was roughly the same as in spiral galaxies.
E) They measured the X-ray emission from hot gas gravitationally bound to the galaxies.

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