Here's a problem to solve: A pharmaceutical company develops a new antibiotic to treat pneumonia. After limited testing, the researchers conclude that the antibiotic seems to be effective. How can the researchers strengthen their conclusions?

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

- A: Test the antibiotic on human volunteers.
 
- B: Publish their results in a medical journal.
 
- C: Ask an independent lab to retest the antibiotic.
 
- D: Test the effectiveness of the antibiotic at killing other bacteria.
 