Answer: A: a historical timeline
Step-by-step explanation:
In describing how smallpox had affected the world and how it was spread, the author gave a brief historical timeline that showed the introduction of small pox, the effects it had and the main events that led to its spread.
The timeline showed that the decline of the Roman empire began with an epidemic and then was spread by successive large scale events such as the Arab expansion which was then followed by the Crusades which were Christian reactions to the Arab expansion.
Small pox also spread to the Americas with the discovery of the West Indies where it decimated the Native American population and along with measles and flu, killed around 90% of Native Americans.