It's possible to roll Miri's head out the airlock while her body blows the reactor core; that will get you ending C. Ending A requires you to obtain all four logs and read the secret fifth log which they unlock.
Which logs are you missing?
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Log 1 is in the same area where you get introduced to the scanner. Look by the door leading to the fight with your first zombie.
Log 3 is in the room that's full of zombies. Note that you can roll Miri's head back out to the first hallway (though you can't follow her body); you can, however, reach the back wall of the room without getting torn apart.