Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Astronomers have found evidence for the existence of a super-massive black hole in the center of the giant elliptical galaxy M87. What is thought to cause black holes to form?
the collapse of a high-mass star
Dense star remnants.