Why did christopher reeve wonder, "what else is there left for superman to do that hasn't been done?

Christopher Reeves was always remembered for portraying the DC superhero Superman as well as a number of roles in movies, television and stage. Yet it was when he suffered a riding accident that left him paralyzed that he showed the world he was truly a Superman.  Reeves concentrated his energy on medical research for spinal injuries that benefited not only him but others.  He continued to push for more research in government until his death.  He has done a lot yet even in the end, he felt there was more to do despite his difficulty.   Even in real life, he truly was Superman.

i would say the correct answer is not concrete.