This is the one that's using Vitamin C as the placebo.
>https://clinicaltrials.gov/ct2/show/record/NCT04328961?view=record
This is a randomized, multi-center, placebo-equivalent (ascorbic acid) controlled, blinded study of Hydroxychloroquine (HCQ) post-exposure prophylaxis (PEP) for the prevention of SARS-CoV-2 infection in adults exposed to the virus.