A ask the rat catchers is usually wise , academia have a history of cleverly missing the obvious
B ask those who have lived with rats in a friendly or hostile setting(i am a vaguely competent ratter due to having ratty de foie gras( RN )and ratatouille (R.R) as shirt pets. very different types of critter
C is it possible that direct human activities have a large effect on rat numbers. climate change will probably get the humans first but city rats will be close behind
im not convinced climate change is the biggest forcing with rats, water(far more for RN than RR), food and secure housing are far above a few degrees warmer or colder on ratty's wish list
humans have been ace at providing them with food and housing(and international travel)
D could fill several pages, i like them as chums and have got quite good at hunting them as vermin