I'm still confused on 20. I know it's not a SA question because the added argument isn't pushing the argument to the point of validity, but I'm still not sure what distinguishes this question from a PSA question. It seems to me if you add the correct answer choice to complete the argument, that's a PSA question, since that's how JY set up the structure to answer these questions. In contrast, strengthen questions are "strengthening the beam" as opposed to adding a structural part to the argument.

What am I missing? Am I getting too hung up on the method for answering these questions?