According to the Textbook,why Did Diseases Such as Smallpox and Measles

According to the textbook,why did diseases such as smallpox and measles produce epidemics that ravaged or completely wiped out native populations in the Americas after contact with Europeans in the late fifteenth century?

A) Europeans refused to share their medicines with the native peoples.
B) The native peoples lacked resistance to these diseases.
C) Europeans forced the native peoples to relocate to massive urban centers,where the bacteria easily spread.
D) Europeans forced the native peoples to abandon their traditional medicines,which had kept the diseases largely at bay before contact.