What union is it ?
This union accepted skilled as well as unskilled workers, blacks as well as whites, and men as well as women. As a result, its membership grew quite rapidly.
2 answers
I don't know. What does your text say? Which unions are options, and when?
I agree with Reed, when? which unions? My guess is it might be focusing on this movement https://en.wikipedia.org/wiki/Congress_of_Industrial_Organizations#History however, there are other union movements that could fit in other time periods. Repost the question if you need more help.