So, apparently, the apocalypse has destroyed every single supermarket on the planet, but the clocks are still running for them to track time this accurately. If the author cares about setting consistency this little, why didn't they just go for a "two girls on a botched field trip" story instead of post-apocalypse?
Well the story falls on more the kind of "what if" scenario that does not have much idea as to how it would actually come about. Kind of like Sounan Desu Ka? where it is a bit iffy about what kind of world the story is based in. Honestly it would be better to forget about the consistency of this story's setting and treat it as a fun educational trip.
Or maybe as @NamelessSentinel stated just take the theory approach as to how this world ended up as what it is. As of now though I have to agree with the theory proposed by @NamelessSentinel since I can't think of how everyone in the world is gone except those two. The only thing I can make out of this is that Aoi and Kia has somehow passed away in some freak accident and now they're both in this world that kinda resembles the after life. Something like Alice in Borderland, minus the violent death games.
