Here's a problem to solve: One year, the oak trees in a park began producing more acorns than usual. The next year, the population of chipmunks in the park also increased. Which best explains why there were more chipmunks the next year?

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

- A: Shady areas increased.
 
- B: Food sources increased.
 
- C: Oxygen levels increased.
 
- D: Available water increased.
 