Simple Random Sample Without Replacement

Note that SRSWOR refers to Let U be a population of size N. From U, we first select a Simple Random Sample Without Replacement (SRSWOR), S_1 , of size n_1 . Then, from S_1 , we select a SRSWOR, S_2 , of size n_2 . Show that S_2 is a SRSWOR of size n_2 selected from U.

I wasn't familiar with the notion of SRSWOR. But during my research I found that the probability of selecting S_1 for example is P(S_1)= 1/[N!/n_1!(N-n_1)!] I also thought that the answer deals with the fact that S_2 depends actually on S_1? I explored the path of conditional probability and proof for inclusion and subset.