Afactor that plays an important role in determining how strenuously and when you exercise

Your weight will be a factor to consider since the heavier you are the more your muscles will have to work to perform an activity that other find easy.

Another factor to consider is the altitude. When you are closer to sea level it is easier to perform physical activity but when you are in higher altitudes, then your body struggles to breath and oxygenate your muscles and therefore you have to take your muscles to the limit.

oxygen +1 most likely

a and c are the answeres