Here's a problem to solve: Different species of herbivorous rodents lived in the same ecosystem. Which factor most likely increased, causing more competition for resources among the rodents?

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

- A: the fertility of the soil
 
- B: the population of producers in the area
 
- C: the amount of food scraps left by predators
 
- D: the rate at which primary consumers moved into the area
 