In 1937, Two German Biochemists Published a Paper Proposing the Following

In 1937, two German biochemists published a paper proposing the following reactions as part of glucose oxidation: citrate $\rightarrow$ isocitrate $\rightarrow$$\alpha$ -ketoglutarate $\rightarrow$ succinate $\rightarrow$ fumarate $\rightarrow$ malate $\rightarrow$ oxaloacetate
Adding succinate, fumarate, or malate to thin slices of tissue increased oxygen consumption, supporting the hypothesis that these molecules are intermediates in the process. However, a puzzling observation was that intermediates were still present in the reaction mixture at the end of the experiment. If they are intermediates, then they should be consumed as the next product in the pathway is formed. What explains this observation?

A) Succinate, fumarate, and malate are not reactants but catalysts, and catalysts are not consumed in the process.
B) Succinate, fumarate, and malate are constantly regenerated in the citric acid pathway.
C) Succinate, fumarate, and malate increase metabolism and therefore oxygen consumption, but they are not directly part of the glucose oxidation pathway.