The most significant reason railroads encouraged industrial growth was
A. the efficient transportation of raw materials
and finished goods.
B. the use of the Bessemer process to build
steel tracks.
C. the increase in labor necessary to build
railroad cars.
D. the connection of the east and west coasts
via railways.

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.

Aor b but i'm going with b