Astronomers believe that the massive object at the center of the Milky Way is a black hole because
A. it is necessary to explain the Galaxy's rotation curve.
B. images show an accretion disk surrounding an empty spot.
C. gravitational wave measurements indicate a black hole is there.
D. observations at X-ray waves, infrared waves, and radio waves limit the size of the object to smaller than 10 AU.
Answer: D
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