@tutor I am still having trouble ruling out AC (C) here.

I feel like C and D both weaken equally and neither fully closes the door on the conclusion.

C attacks this idea that it must be a geological process because we ruled out worms due to multicelled animal life not being around. The issue here is that it could have been some earlier form of single-cell life that created the wormlike marks.

To me, this is a very clear gap in the argument. We say it can't have been multicellular animal life so worms are ruled out, so AC (C) introduces the possibility of other forms of life causing the marks, attacking the assumption and gap that this must lead to it being a non-life cause.

Sure, hard to distinguish does not mean can never distinguish, and sure it does not give you a time frame, but it OPENS UP DOUBT to the idea that perhaps neither the geological process or the worms were the cause.

AC (D) has this same issue, what if the sandstone had been moved from where it was found from where it was marked? The AC also says "geological processs that are likely" what about geological processes that are not likely? "Not likely" still means "possibly."

My point is neither AC weakens in a way we can ostensibly say is "more weakening". I truly am at a loss for words at my frustration with this question. I feel like it breaks everything I understand about how were supposed to evaluate arguments and answer choices and I genuinely feel like this is a poorly written question.

If anyone can help me understand why C is ostensibly weaker than D, even though both require assumptions. I truly believe the testmakers equivocate with respect to this central idea of life forms and, in calling C a wrong answer, unironically commit an error of logical reasoning by equivocating all life forms with multicelled life forms.