What factor contributed the most to the depression of 1920-21
a)speculation
b)demobilization
c) foreign imports
d)tariff reductions

It was stock market "speculation" that contributed the most to the Depression of 1920-21, since the value of many companies became incredibly inflated--leading to a "bubble" that eventually burst in 1929. 

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.