Answer:
hi how are you doing today Jasmine
The air outside your body because when you tried to hold your breath the pressure builds and forces your body to breath by letting out the air but depending the altitude you are the pressure can change hope this helps
If the stimulus reaches the threshold an action potential will be generated. If the stimulus doesn't reach it no action potential will be generated. If the stimulus will be stronger the action potential will have the same amplitude. The refractory period makes it harder to generate an action potential so they decrease the frequency of action potential production.