I think it's A
the cell membrane contain protein molecules
They are changed because of the environments changing
One of the most general forms of discomfort or pain one feels at the time of strenous work out is a burning sensation in the muscles or lungs, which goes away after some time, that is, after stopping the activity. This is a result of an accumulation of lactic acid.
Lactic acid is a by-product of the procedure the body goes through when it requires to generate energy more briskly that it does usually, like when one exercises.
The muscles functioning generally produce energy aerobically, that is, by using oxygen, however, when one push himself or herself at the time of workout and enough oxygen is not accessible, then these muscles start producing energy anaerobically, resulting in production of lactic acid as a by-product and ultimately causing burning sensation.
