Left on its own, the sa node would normally keep your heart rate at when you are resting, your heart rate is slower than that because of

The first one is Pulmonary
The second one is Systemic

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)

Ithink the answer is d.