PT 28, S4, Q15

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It's an article about 2 schools of economics. I don't understand Q15 which answer is A. I chose B.
A. The environment's ability to yield raw material is limited.
B.Natural resources are an external constraint on economics.

Isn't that A can be interpreted as that natural resources is limited, and that's what Line 20-22 says (steady-state economists' view) and Line 30-35 saying "that natural resources, if depleted, can be replaced by'' shows that the neoclassical economists also believed that natural resources is LIMITED (although it could be replaced by other resources)?
And I don't see why B is wrong. Although the passage doesn't explicitly say that the steady-state economics hold this conception (or hold the conceptions like external/internal constraints.)

Can anyone explain this?

Many thanks!

LSATisland

I took the LSAT already and no longer have my PTs. But if you message me the details of the question, I'd be happy to help.