How does the book of Philemon, a short letter from Paul, challenge or subvert the social norms of the 1st-century Roman Empire?

A By advocating for the complete abolition of slavery throughout the empire. B By urging Philemon to free his slave Onesimus and treat him as a brother in Christ, indirectly challenging the master-slave relationship. C By establishing a formal process for slaves to gain their freedom through the church. D By primarily focusing on the economic benefits of treating slaves more humanely.