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

A God is like the unjust judge and needs to be worn down with constant requests. B Prayer should be offered only when one is in desperate need. C God will bring about justice for his chosen ones, who cry out to him day and night, even though he may delay. D Widows are especially favored by God and their prayers are more effective.