The few individuals of the population where immune to a certain disease. The desease killed the rest of the population, and only some (immune individuals) remained alive.
The survivors, reproduced giving birth to offspring with the same resistance, making the whole/big part of the population resistant to the desease (the resistance was inherited from the parents).
Answer:
[tex] \boxed{\bf~Option~B.~is~correct.} [/tex]
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