Which pair of sentences best describes a scientific discovery in A Black Hole Is NOT a Hole by Carolyn Cinami DeCristofano?

First, scientists observed a black hole through a telescope. Next, other scientists used research to explain what caused it.
First, scientists went searching for black holes in the universe. Next, the same scientists discovered a supermassive black hole.
First, scientists studied radio waves coming from space. Next, other scientists used their discoveries to identify black holes.
First, scientists studied radio waves coming from space. Next, the same scientists discovered black holes.