Which Occurrence Has NOT Been Observed as Evidence of Colliding

Multiple Choice

Which occurrence has NOT been observed as evidence of colliding galaxies?

A) very strong X-ray emission coming from rich clusters of galaxies
B) starburst galaxies that contain hot gas, many newborn stars, and relatively young globular clusters
C) streams of hydrogen gas connecting two or more galaxies
D) distribution of black hole candidates throughout a galaxy suggesting many star mergers as a result of a galactic collision