Here's a problem to solve: Which of these indicates a physical change that occurs during the process of digestion?

Among the 4 following options, which is the correct answer?

- A: the addition of acids to create a ball of food
 
- B: the secretion of pepsin to change proteins to peptides
 
- C: the breakdown of food substances by digestive juices
 
- D: the squeezing of food through the intestines
 