C

Non-competitive antagonists bind to an allosteric (non-substrate) site, so they cannot be overcome by adding more agonist: the maximal effect (V max) falls, while the affinity of the agonist for its site (Km) remains unchanged, yielding a Lineweaver-Burk plot with decreased Vmax and unchanged Km [Gill, 1990, PMID 2370090; Berrettini, 1978, PMID 281895].