Why is the spread between the upper- and lower-income groups so difficult to shrink?为什么高收入阶层与低收入阶层之间的差距这么难以缩小?The agreement included a stipulation that half of the money had to be spent on housing for lower-income families.协议中包括一项条款，即这笔款项中的一半必须用于低收入家庭的住房供给。