Why Did the Plague Spread So Rapidly on the European

Why did the plague spread so rapidly on the European continent in the mid-fourteenth century?

A) The bacterium that likely caused the disease, Yersinia pestis, followed major trade routes to and across the continent.
B) Invading Ottoman troops spread the disease to Europe during their siege of Constantinople and their attacks on the Balkans.
C) The fighting during the Hundred Years' War allowed the disease to erupt among wounded and exhausted soldiers.
D) Europeans had yet to learn about quarantines or other measures that could effectively halt the spread of disease.