The zombies were increasing in number.
Brainless zombies should be easy to deal with. As long as one put their mind to it, humans with organization and weapons should be able to clear them out in due time.
Luring the zombies into a trap and using fire to deal with them, of course, that's possible. Lure them into a canyon, bury explosives to detonate, set fires, sweep both sides with machine guns, and if that doesn't work, bombard them with cannons or crush them with tanks.
All of these methods are feasible, but there's just one problem.
How to control the number of zombies lured out?
If there were only a hundred zombies, a thousand, or even ten or a hundred thousand, it would be easy to deal with—too easy, in fact.
But what if there were at least five million zombies in a city? How could one deal with that?
What would be the point of luring zombies out of the city one by one? And if a large number were lured out, then what to do when there were too many?
