CASE HISTORY in 1884,Yong Ding Was a 38-Year-Old Male in Canton,located in Canton,located

CASE HISTORY
In 1884,Yong Ding was a 38-year-old male in Canton,located in southern China.Life was hard,but Yong Ding,a cook,still managed to support his family.As he walked to his restaurant each day,he barely noticed the small bands of rats scurrying through the streets.Disease was rampant that year;victims of the Shuyi (rat epidemic) were stacked like firewood in the streets,five bodies high in places,waiting to be taken to burial.Yong knew that the disease started as a swollen gland in the armpit and often had a black appearance (now called a bubo) .He checked himself daily for these swellings and,seeing none,always felt relief.Then,one morning,he found one.Within days,Yong Ding began coughing blood as the agent (a mystery at the time) spread through his bloodstream to his lungs.Once that happened,Yong Ding knew death was not far behind.More than 60,000 died this way in what was to be the start of the Third Pandemic of bubonic plague.Yong Ding's body was one of many lining the street that year.
We now know that the cook Yong Ding was living through the Third Epidemic of bubonic plague.We also know that the rats that scurried randomly through the streets were infested with fleas,which carried the causative agent of the bubonic plague: Yersinia pestis.In this complex infection cycle,which organism was the vector?

A) Yersinia pestis
B) rats
C) fleas
D) humans