单选题 Although at the______these schools were attended by only a tiny percentage of the population, numbers increased during the 19 th century as waves of immigrants entered.
【正确答案】 B
【答案解析】解析：setback挫折；outset开始；outlet出口；sequence顺序，关系。句意是，虽然学校开始只有很少的人参加，但在19世纪随着移民的进入，人数不断增加。所以正确答案为B。