A randomized controlled trial is conducted to evaluate the incidence of diarrhea in 300 children with otitis media treated with a broad-spectrum antibiotic, amoxicillin clavulanic acid, compared to amoxicillin. During the study, the participants and the investigator were not aware of the treatment allocation between experimental and control groups. This study is:
A. Open-label
B. Single-blinded
C. Double-blinded
D. Non-blinded