第一段:Prior to 1975, union efforts... were limited, 解释limited 原因。
第二段:1970 mid之后的一个现象。
(1,2段结合起来是一个大现象)
第三段:解释现象。TS:The absence of any comparable increase in unionization among private-sector clerical workers, however, identifies the primary catalyst—the structural change in the multi-occupational public-sector unions themselves.