By the time the embryo is weeks old, all the main internal organs have formed, along with major external body structures. a. two b. four c. six d. eight

Ibelieve the answers are a and c.

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)