Here's a problem to solve: A student wants to determine the effect of garlic on the growth of a fungus species. Several samples of fungus cultures are grown in the same amount of agar and light. Each sample is given a different amount of garlic. What is the independent variable in this investigation?

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

- A: amount of agar
 
- B: amount of light
 
- C: amount of garlic
 
- D: amount of growth
 