The Crab Nebula Is an Important Test of Our Ideas

The Crab Nebula is an important test of our ideas about supernova explosions because

A) people saw the supernova and later astronomers found a pulsar inside the nebula.
B) the system contains an X-ray binary.
C) the nebula is expanding slowly, as expected from mass-loss rates in massive stars.
D) the original star must have been like the Sun before it exploded.
E) astronomers observed the merger of the two stars.