Answer:
Option c.
Explanation:
When the Native Americans first came across the Europeans, the past American period of limited diseases terminated with the arrival of the Europeans.
This can be understood in a way that when the Europeans were not there in the past the population as compared to Europe was limited and as the Europeans came , these interactions and the population led to the spread of deadly diseases like that of syphilis which spread widely as the Native Americans had not developed the immunity against such diseases due to continental isolation for quite a long period.