In the Parable of the Persistent Widow (Luke 18:1-8), what is the central theological point Jesus is making about prayer, beyond simply encouraging perseverance?

A God always answers prayers immediately and in the way we expect. B God is unjust and requires constant nagging to grant our requests. C God, unlike the unjust judge, is inherently just and will ultimately vindicate His elect who cry out to Him day and night, even if the answer seems delayed. D Prayer is primarily a means of convincing God to change His mind.