Read this excerpt from A Black Hole Is NOT a Hole by Carolyn Cinami DeCristofano.

In fact, every quasar and radio galaxy dot turned out to be energy from the center of an entire galaxy. Astronomers found stars careening around these centers, zooming at previously unheard-of speeds in their orbits. The motion was like an accelerating ice skater pulled around her partner in a tight twirl—or like a fish caught in a superstrong whirlpool. Something extreme had to be in the middle of each of those galaxies.

You might be able to guess what this was, but for scientists, the best answer came as a shock. In 1994 telescopic evidence confirmed the amazing cause: a supermassive black hole, with the mass not of one imploded star, not of two, but of millions—maybe even billions.

In the excerpt, the author makes a connection between the discovery of quasars and radio galaxies and

the invention of the radio telescope.
the scientist who mapped radio waves.
the identification of the Milky Way.
the confirmation of a supermassive black hole.