How does the apostle Paul, in his letter to Philemon, subtly challenge the social norms and power structures of the Roman Empire regarding slavery?

A By explicitly condemning the institution of slavery and demanding Onesimus' immediate emancipation. B By advocating for a gradual reform of the Roman legal system to eventually abolish slavery. C By appealing to Philemon's love and Christian principles to receive Onesimus back not as a slave but as a brother in Christ, thereby undermining the master-slave relationship. D By threatening Philemon with divine retribution if he does not comply with Paul's demands to free all his slaves.