In the Parable of the Persistent Widow (Luke 18), what is the main point Jesus is trying to convey about prayer?

A God only answers prayers from those who are truly needy. B God is like the unjust judge and needs to be worn down by constant requests. C We should always pray and not give up, trusting that God will hear and answer. D Prayer is only effective if it is done in public, where others can witness our faith.