Question 20·25 Super-Hard SAT Transitions Questions·Expression of Ideas

That failure alone was inconclusive limits what the first test proves, and we need to link that limit to the isotope finding. Why wasn't the failed conversion enough? L might have needed a short-lived reagent generated only while earlier steps were occurring, so testing it separately could have left out something it needed. The first test leaves L's role open.

What does isotope tracking add? The finding that the atoms incorporated into the target had bypassed L means L wasn't a step on their route to the target. That's why the result is incompatible with L's proposed role: it rules out the proposal rather than leaving it open. Both tests question L's role, but the new finding settles what the first left open, so it gives stronger evidence for the same conclusion.

Now swap in the choices' plain meanings. Similarly means in the same way, which misses the move from an uncertain test to a finding that rules out L's role. On the contrary means the opposite is true, but isotope tracking doesn't deny the first test's limit: that test alone was inconclusive. For example means here's one case of what was said before, but bypassing L doesn't illustrate why L might have required a short-lived reagent.

More decisively means with evidence that settles the question more firmly. At the blank, it introduces the atoms' route as firmer proof against L's proposed role, which is what the isotope finding adds. Choice A.