Which is a strength-training activity? sit-ups tennis hopscotch jump roping

Sit ups, because you are using your core.

Sit ups, because you are using your core and strengthing your stomach

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)