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Hi,
I'm having a lot of trouble eliminating D on this question, and would love some help if possible! My reasoning:
D) Was the species to which the recently discovered [earlier] dinosaur [X] belonged related to T. rex?
If yes, and T. rex descended from X, this seems to strengthen the author's argument as it suggests that the T. rex features evolved in an earlier ancestor that evidently did not require them for its size. In this scenario the features would have just been passed down to the T. rex, offering an alternate explanation to the scientists' hypothesis and strengthening the author's argument.
If no, and the T. rex and X were unrelated, I feel like this would weaken the author's argument as it would make it difficult to compare the two dinosaurs. Even if X did not evolve these features to support its size, T. rex still may have and it's just a coincidence that they are similar. This would leave us back where we started as the author's evidence for calling the scientists' hypothesis into question is now irrelevant.
Any help clearing this up would be greatly appreciated, I've spent way too long on this question :P