Why can we not measure the mass of elliptical galaxies using the 21-cm line of hydrogen gas?
A) All elliptical galaxies are too far away for us to detect the 21-cm line.
B) Supermassive black holes in the center of elliptical galaxies disturb measurements of the 21-cm line.
C) The disorderly motion of the material in elliptical galaxies prevents us from measuring different velocities at different sides of the galaxy.
D) Elliptical galaxies have little gas, so do not have strong 21-cm line emission.
D
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