Which best explains why collective bargaining increases the bargaining power of workers?

In the above question, options are not given so the answer has been generalized.

Collective bargaining power gives rights to the workers to put their demands in front of employer in a better way. It puts the workers and employers on the same level.
Collective bargaining increases the power of bargaining of workers. If the workers and employers have agreed on a collective bargaining on mandatory subjects then it is unfair for the employer to refuse. 

Yes it would be a physical change.

What are the choices?