Although Diseases Were Widespread in North America,colonists Were Less Susceptible

Although diseases were widespread in North America,colonists were less susceptible because:

A) of the dispersed nature of the American population.
B) Americans had been exposed to most diseases in Europe and had built up an immunity.
C) the colonies had much better doctors than England.
D) the diseases themselves were not as virulent as their European cousins.
E) American cities were cleaner than their European counterparts.