一道SAT语法的ISE题目请教

问题描述:

一道SAT语法的ISE题目请教
In the 1960s,Cesar organized migrant farmworkers,(many of them) Mexican Americans,into a labor union that supported nationwide boycotts of both lettuce and grapes.
我有疑问的划线处就是括号里的内容.
此句子是No error的.可是many of them没有错么?我觉得应该是many of whom were吧~各位有见到过那种表达么?

这句话确实成立,many of them Mexican Americans 是独立主格结构,作伴随状语,补充说明人员组成情况,省去了现在分词 “being”;完整表达:...many of them (being) Mexican Americans,...但楼主的“...many of who...