I think that you certainly can use previous work from scenarios, but there is a slightly better approach for a must be true question like this that does not introduce a new rule. These are testing something that is always, universally true, so ideally we would have it in our initial diagram. If you see a plain must be true question like this, and you did not make any of the answer choices as a deduction, that suggests that there was a deduction you missed on the initial setup.
