Please choose the correct answer to the question: 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?

Answer1: the death of a low-mass star Answer2: the gravity of a large nebula Answer3: the collapse of a high-mass star Answer4: the combining of material from planets

Answer format: answer1/answer2/answer3/answer4