Why would anyone kill the entire population except 3 people?
Theoretically, if an asteroid fell to Earth and inside it was a device spreading a toxin that would kill the entire population except for 3 people, and these 3 people were to have something that the people who killed humanity needed to create an antibiotic, for example, from their bone marrow, how would you explain why these 3 people? Why did they have to get them through selection? Why wasn't there another way to do it? Would they really need bone marrow or something else?