Why not A?

The author here is refuting the idea that oil might have formed from inorganic carbon deposits, because we found bio markers, which are a sign of life, in oil. To weaken this we would ideally want to show that bio markers also might show up in inorganic things, or somehow else got into the oil despite the oil not being organic. A shows that not all organic things have biomarkers, which is different from saying that biomarkers can appear in inorganic things. If we had been told that some non fossil rocks had biomarkers that would be great, but the fact that they sometimes fail to appear wouldn't explain why they show up in a place we wouldn't want them. I agree that A shows biomarkers aren't reliable, but it does so in the wrong direction. It shows that biomarkers are too weak to prove something is organic, when what we needed is that they are so strong that they can appear even when inorganic.

Ah yea I see that. Thank you Emil.