According to Jared Diamond, Why Were the Germs Brought by the Spaniards

According to Jared Diamond, why were the germs brought by the Spaniards so much deadlier to the Incans than the Incan germs were to the Spaniards?

A) The Spaniards had developed cleaner habits than the Incans.
B) The Incans lived closer together, so the Spanish germs were transmitted far more easily.
C) The Spaniards had lived in close proximity to domesticated animals for much longer than the Incans.
D) The large Eurasian continent allowed the Spaniards to be exposed to a larger variety of germs than the relatively smaller South American continent of the Incans.
E) The Spanish diet contained more antibiotic agents than the Incan diet.