Read the following passage: "The concept of a 'black hole' arises from Einstein's theory of general relativity, describing a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The boundary of no return around a black hole is called the event horizon. While black holes do not 'suck' things in like a vacuum cleaner, any matter or light that crosses the event horizon is inevitably pulled towards the singularity at the center. Black holes are formed from the gravitational collapse of massive stars, and they can range in size from a few times the mass of our sun (stellar black holes) to millions or billions of times the sun's mass (supermassive black holes found at the centers of galaxies). Despite their name, black holes are not empty space; they contain an enormous amount of mass packed into an incredibly small volume." What is the primary characteristic that defines a black hole, according to the passage?

1. AIts ability to 'suck' in nearby matter and light
2. BBeing composed of empty space at its core
3. CIts formation exclusively from stars similar in size to our sun
4. DA region of spacetime from which nothing can escape