What is the relative pronoun of "to whom the poet is referring to"

The reversalists continue their labors, confident that they will one day construct an engine that generates more compression than it uses, a perpetual power source that will restore to the universe its lost vigor. which theme is best supported by the excerpt? a. scientific study requires dedication and perseverance. b. people will always look for a way to achieve immortality. c. death must always be endured alone. d. change is difficult to accept but beneficial if embraced.