What proved to be the major stumbling block for germany in russia?

You're talking about World War II, right? Well, yeah, at one point Germany attempted to invade the Soviet Union (specifically during the Battle of Stalingrad.) But a major stumbling block was the weather in the Soviet Union; it was fiercely cold and German soldiers were starving and sick. Eventually, it became so cold that the German military retreated and Hitler's plans for "world domination" officially ended. 

high entry costs prevent new producers from entering the market.

explanation:

an oligopoly is a term used to clarify the structure of a particular market, industry, or organization. a market is considered oligopolistic or amazingly focused when it is shared between a few companies.

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.