Here's a problem to solve: Derek hypothesizes that a potted fern in a closed container will stay healthier than the same type of plant on an open windowsill. At the conclusion of his investigation, Derek finds that his results support his hypothesis. Which best explains why Derek should repeat the investigation?

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

- A: to be able to start another investigation
 
- B: to prove the investigation is important
 
- C: to learn why the hypothesis is correct
 
- D: to make sure the results are valid
 