GMAT逻辑OG12-71问题!

GMAT逻辑OG12-71问题!71.Roland:The alarming fact is that 90 percent of the people in this country now report that they know someone who is unemployed.Sharon:But a normal,moderate level of unemployment is 5 percent,with one out of 20 workers unemployed.So at any given time if a person knows approximately 50 workers,one or more will very likely be unemployed.Sharon’s argument relies on the assumption that(A) normal levels of unemployment are rarely exceeded(B) unemployment is not normally concentrated in geographically isolated segments of the population (C) the number of people who each know someone who is unemployed is always higher than 90 percent of the population(D) Roland is not consciously distorting the statistics he presents(E) knowledge that a personal acquaintance is unemployed generates more fear of losing one’s job than does knowledge of unemployment statistics.为什么选B?C哪里不对?