Answer: A — Spanish soldiers
Smallpox reached America through Spanish soldiers (conquerors), whose germs decimated native populations that had no immunity against European diseases.
Source: The Making of a Global World, Section 1.3 — Conquest, Disease and Trade
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The passage clearly states: "the most powerful weapon of the Spanish conquerors was…the germs such as those of smallpox that they carried on their person." The key word is Spanish conquerors/soldiers — not French, Portuguese, or British. Examiners look for this specific detail.