(A) Poverty and diseases
Until the nineteenth century, poverty and hunger were common in Europe, cities were crowded, deadly diseases were widespread, and religious dissenters were persecuted. Thousands therefore fled to America.
Source: Chapter 3, Section 1.3 — Conquest, Disease and Trade
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The textbook explicitly states: "Until the nineteenth century, poverty and hunger were common in Europe. Cities were crowded and deadly diseases were widespread… Thousands therefore fled Europe for America." Option (B) refers to the California Gold Rush (not the primary reason stated in the text), Option (C) relates to an earlier period, and Option (D) is completely anachronistic (Vietnam War is 20th century). Always quote the textbook reason directly in MCQ justifications if asked to explain.