In Both the Labeled-Release and Pyrolytic-Release Experiments Conducted on Mars

In both the labeled-release and pyrolytic-release experiments conducted on Mars by the Viking Landers, carbon compounds were fed into the system and then this carbon was searched for after being potentially involved in respiration or photosynthesis. But how would we know it was the SAME carbon atoms?

A) Atoms produced on Earth are distinct from those produced elsewhere in the universe.
B) Carbon is not part of the Martian composition, so any carbon detected would have to be part of the original sample.
C) The released carbon was a radioactive isotope and was thus easily traceable.
D) What was done was to measure the total volume of carbon in the sample. Any increase in this volume would indicate that the sample had stimulated the production of additional carbon.