You only have to do one of the paths to progress Carla, Lina probably being the easiest.
Alex though... I remember you can burn her in the forest if you push her head down while getting a blowjob. I think you can also burn her if you try to tell her you love love her at corruption 3? Another way I believe is telling her you don't want babies after getting her pregnant. Though that ones a little sad.
The game doesn't check for a specific burn, just that the character was burnt and that you asked Carla about them.