The oxidative decarboxylation of pyruvic acid to acetyl CoA occurs in the

Explanation

Pyruvate is carried across both mitochondrial membranes and is then acted on by the pyruvate dehydrogenase complex in the matrix, which removes one carbon as carbon dioxide and reduces NAD to NADH. Because this step needs oxygen further down the chain to keep NAD supplied, it stops in anaerobic conditions while cytoplasmic glycolysis continues.