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The C-O abundance ratio is a powerful probe of galaxy evolution comma sensitive to gas outflows and star formation histories because carbon and oxygen enrichment occurs on different timescales. C-O has been derived for many low- and high-redshift galaxies using measurements of rest-UV OIIICORCHETE_CLOSE and CIIICORCHETE_CLOSE collisionally excited lines (CELs). However comma C-O derived from UV CELs may be subject to an V1.0 and potentially large bias. It is well established that O-H determined from CELs systematically underestimates O-H based on O recombination lines (RLs the most robust abundance determination) by a factor of virgul2 comma known as the Abundance Discrepancy Factor (ADF). It is currently V1.0 whether C abundances display a similar ADF between CEL and RL methods. Consequently comma we do not know whether C-O derived from UV CELs matches the true C-O value based on RLs comma or is systematically biased. The primary obstacle is a lack of overlap between samples with measurements of ultra-faint optical RLs and UV CELs. We propose to measure UV OIIICORCHETE_CLOSE and CIIICORCHETE_CLOSE CELs in 9 HII regions with existing measurements of OII and CII RLs comma leveraging the unique capabilities of HST-COS. These observations will crucially allow us to derive C-O and C-H from both CELs and RLs for the same objects comma enabling a robust calibration of the C-O abundance scale from UV CELs and an understanding of the ADF of C. Securing UV spectroscopy with COS is the only feasible way to validate UV-based C-O comma of critical importance to understanding rest-UV line measurements in local dwarf galaxies comma galaxies at cosmic noon comma and sources in the epoch of reionization with JWST in the near-future.