Most 'humane' way to kill feeder rats? Liquid nitrogen or CO2?

Hello! I'm new here, this is my first thread. I want to know what is the most humane way to kill a feeder rat, to afterwards freeze it. I read a lot about C02, then fell on a comment that said liquid nitrogen was the most painless way to kill feeder rats because it makes them unconscious, then kills them, whereas CO2 hurts their lungs, and they suffocate to death?
Let me know how each works, which hurts them less, how quickly each kill them... And anything else you know. Thanks!!