Sentence Correction (SC) | As a result of medical advances, many people that might at one time

Hi
GMATNinja ,mike

I am still confused with "might have once died" and "might have at one time died" in the answer choices.
Is it not true that usage of "might" itself suggests that the occurrence is highly unlikely? If that is the case, then "might have once died" and "might have at one time died" both indicate events that were highly unlikely to occur.

Also, is "at one time" not equivalent to "once" ?

" many people
...

---