Why do astronomers think that neutron stars were formed in supernova explosions? Choose one: A. Pulsars are made of heavy elements, such as those produced in supernova explosions. B. Pulsars sometimes have material around them that looks like the ejecta from supernovae. C. A pulsar spins very rapidly, as did the massive star just before it exploded. D. All supernova remnants contain pulsars