This is the route you'll see in your first playthrough, just do whatever and enjoy the incredibly abrupt ending

This route only unlocks after you've already played route A, to get this route:

Don't reject Dog-Girl

Roast the seagull

From there you should be on track to see the route B ending, although it appears that there's a Route Bb and a Route Ba, since upon clearing it the game tells you which one you got, but since I only got Route Bb I can't clarify the differences, the game only has one Route B achievement anyway

Thanks to Dream, I now have some extra information on the other routes.

Route C is accessed by picking all the nicest dialogue options possible,

Rejecting Dog-girl does lead to a difficult version of Kill the deadline which ends up with a literal dead end

Route D is accessed through Noisz Re||Verse