Hey
what subtance in tobacco products that causes stained teeth and skin?

Nicotine in tobacco products cause stained teeth and skin.

The answer is b. increased blood flow reduces nerve sensitivity

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)