In the Parable of the Persistent Widow (Luke 18:1-8), what is the main point Jesus is trying to convey about prayer?

A God will always grant our requests immediately if we ask persistently enough. B It is important to publicly display our piety to influence others. C That believers should pray without ceasing and never give up hope, even when faced with apparent indifference or delay. D God only answers the prayers of the righteous.