Which word correctly completes the sentence?

when the choral group began, the voices carried throughout the auditorium.

a.
singers'

b.
singers

c.
singer's

samuel gompers was an extremely important person in the american labor movement, being him an employee all his life he devoted himself to fight for the rights of his pairs, he believed they all had the right to have better thing and that they all were working for them but the factory workers were greedy, ambitions and socialist people for him who he didn't have a good concept of.

he gave so many speeches and led so many strikes and protest to fight for equality which he considered employers didn't want to accept, so he had an absolutely negative opinion of them.