Please help with APUSH question. Really appreciate it. "The only encouragements we hold out to strangers are a good climate, fertile soil, wholesome air and water, plenty of provisions, good pay for labor, kind neighbors, good laws, a free government, and a hearty welcome. The rest depends on a man's own industry and virtue."

"If a European has previously resolved to go to the western country near the Allegheny or Ohio rivers,. .. a few day journey will bring him to Cumber­ land... from whence the public road begun by the United States, crosses the mountains

"You will, however, observe that the privilege of citizenship is not granted without proper precautions; to secure that, while the worthy are admitted, the unworthy should, if practicable, be rejected. You will from hence deduce the importance of good moral habits, even to the acquisition of political rights."

-Clements Burleigh, Shamrock Society of New York, "Advice to Emigrants to America," 1817

Which of the following reinforced the message that Burleigh was sending to people who wanted to move to America?

(A) American System

(B) Industrialization

(C) Protective tariff

(D) Rush-Bagot Agreement

Please help me with this question. I know the answer to this question (A. American System), however, please explain to me as to why that is the answer. I would really appreciate it.