What Is Remarkable About the Supernova Remnant Cassiopeia A

What is remarkable about the supernova remnant Cassiopeia A?

A) Cassiopeia A contains a binary neutron star system in which one neutron star is significantly more massive than the other.
B) Cassiopeia A is bright at all wavelengths from radio and visible light to X-rays.
C) Cassiopeia A is the nearest supernova remnant, located only about 300 ly from Earth.
D) Cassiopeia A shows that a supernova occurred in the Galaxy about 300 years ago, but there is no record of any supernova having been seen at that time.