Read and reread the excerpt from We Shall Not Be Moved. The officers of Local 25 sent them on to the owners — along with one more demand, recognition of the union. This came to mean the "closed shop." Only union members can work in a closed shop, since the only way to set uniform wages and standards is to have the union do it; otherwise employers would always try to undercut one another. And the union can standardize the industry only when it controls the supply of labor through the union shop.

What is the meaning of a “closed shop”?

A “closed shop” is a factory where all of the workers are union members.
A “closed shop” is a factory that hires both union and non-union members.
A “closed shop” is a factory where the owners are in control of setting wages.
A “closed shop” is a factory where the owners control the supply of labor.