Is an oligopoly market structure good or bad for society?

a$.$

Oligopoly is bad for society because it always creates deadweight loss.

b$.$

Oligopoly is good for society because it never creates deadweight loss.

c$.$

Oligopoly is good for society if firms collude, but bad for society if they do not.

d$.$

It is bad for society if firms collude, but similar to perfect competition otherwise.