Which of the following statements best summarizes the existence of supermassive black holes?
A. Supermassive black holes occur in most, if not all, large galaxies.
B. The existence of supermassive black holes is hypothetical.
C. Supermassive black holes are extremely rare; there are only a handful in the universe.
D. Supermassive black holes can only exist in the space between galaxies.
