Hi Sherry!
The conditional statement in this stimulus is the conclusion of the argument (the second half of the last sentence), as Beth already diagrammed. So one error with the logic you've diagrammed is that it doesn't represent the conditional statement given in the stimulus. Remember to use those sufficient and necessary indicator words when they are given. "If" is a common sufficient indicator and "must" is a common necessary indicator.
Once you identify that conclusion and see that it is conditional, you should focus on it. There's something mentioned in the conclusion that is "new" information: the idea of "to prosper." The key here is not about whether or not the theories are
accurate. It's about whether they can help their nations
prosper. That's the key component missing from your logic: why should they look beyond the borders? It's not just because theories based on idealizations are less accurate. Looking beyond borders is necessary for a nation to prosper.
Another thing to keep in mind: this is a Justify question. The stem says "follows logically IF assumed." When you have "if assumed," it isn't an assumption question and "the conclusion follows logically" is a common phrase in Justify question stems. Since this is a Justify question and not an Assumption question, you can't use the Assumption Negation technique (it only works for Assumption questions!).
So we should be using the Justify Formula:
Premises + Answer Choice = Conclusion
Premises: EESI
ELBB
(this is the contrapositive of what the premises basically say, that if economists aren't looking beyond borders, then they aren't taking into account every significant influence)
Answer Choice (A): NP
EESI
(if nation is prospering, they must examine every significant influence)
If you link the Premises and Answer Choice (A) together, you get:
NP
EESI
ELBB
Which brings you to....
Conclusion: NP
ELBB
(if nation is prospering, economists must look beyond borders)
So the addition of answer choice (A) proves our conclusion. Answer choice (C) is not nearly strong enough to prove that for a nation to prosper, the government economists need to look beyond their borders.
Hope that helps!
Best,
Kelsey