i already said fire would do the job to cleanse all rats.
No it wouldn't, you're severely underestimating how frequent they are. They'll be like several hundred generations per week (gross understatement) while people all around the globe coordinate this fire plan.
so, a big fire would took them all out. you are underrestimaiting the power of it.
No, if anything, you're underestimating the power of the fire. If we're making a fire strong enough to obliterate rats everywhere then people are inevitably going to get hurt. Plus you'd be killing other organisms too in the process.
so, science would find a way to burn out all those rats that are living in human's places first, and then burn out those that are close to human. as for the remaining rats over other areas of the world. science would also find a way to exclude them from us.
And how would they manage to do that before we go extinct considering that we've been trying for decades to kill rats and haven't been successful? It would likely only take a couple years for us to go extinct... <_<
3 years isn't anywhere near enough time to solve a problem we've been trying to solve for literally decades and haven't made much progress...
because we never put like really really really big amount of money to develop machinery and medicine that can kill most rats easily.