Chemical digestion happens with the of that are sensitive to ph.
a. organs
c. cells
b. enzymes
d. muscles

The answer is letter B. enzymes.

Digestive enzymes are the ones responsible in breaking down macromolecules into tiny and manageable pieces that can be easily processed by the digestive system. They play a crucial role in facilitating the absorption of nutrients within the body and can be found within the digestive tracts of animals and humans. In the stomach, it is secreted by the lining of the stomach which in turn produces juices that are necessary to break down the food before it passes through the intestines.  

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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)