C. Give reasons for the decline of Yuan Dynasty.

In general, there were two major factors that contributed to the Yuan Dynasty's demise: one was class conflict caused by heavy taxation, and the other was ethnic conflict caused by the 'Four Class System.'Also, power struggles within the ruling class became more and more serious. The final years of the Yuan dynasty were marked by struggle, famine, and bitterness among the populace.

The constant presence of diseases or infectious agents within a given geographic area or population group. it may also refer to the usual prevalence of a given disease with such area or group. it includes holoendemic and hyperendemic diseases. a holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). a hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. so probably family history.