How Do Astronomers Know That There Is Activity in the Nuclei

Multiple Choice

How do astronomers know that there is activity in the nuclei of some galaxies?

A) Their nuclei are much brighter than expected.
B) The orbits of stars near the centre of the galactic nuclei are faster than expected.
C) The luminosity of the nuclei varies over time.
D) The colour of the nuclei is different than expected.