Final answer:
The likely factor causing the change in the paramecium's numbers is probably increased food availability or an environmental shift that affected their growth or reproduction.
Step-by-step explanation:
The factor most likely accounting for the change in the paramecium from 8 to 20 hours is most probably food availability or a change in environmental conditions that affected the paramecium's reproductive or growth rate. In biology, changes in an organism’s environment or the availability of resources directly influence its life cycle and reproduction. For instance, greater food availability or decreased predation can lead to a population increasing more quickly, and these conditions tend to have immediate rather than evolutionary effects on population changes.