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Taaaa76 they would have a selective advantage, since they more likely to survive. However, because a random event has occurred (i.e. heatwave) and has reduced the genetic diversity of the population, the bottleneck effect/C makes the most sense. Yes, natural selection is almost always going to be present, but in this case, it's not the main thing that has occurred. Often with exam style questions, there is a somewhat right answer (i.e. A) but also a more correct answer (i.e. C). Hope that helps!