I’n pretty sure the answer is supposed to be about the tectonic plates. The plates constantly shift the land creating holes/dips in the earth and mountains in other places.
The right answer is smelly garbage.
Infestation is the penetration into the body of a parasite whose nature is not microbial.
smelly garbage is not synonymous of infestation, because the olfactory system only detected smelly metabolites of these microorganisms. This does not necessarily mean that the microorganism has managed to penetrate the body.
By cons other proposals are signs and tools for infestation (torn packaging with food leaking out, dead insect bodysuits, feathers, and fur).
If the chemical touches another chemical, then there is a higher probability that it's going to react.
