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

A God will only answer prayers if they are repeated many times. B God is like the unjust judge and needs to be worn down before he acts. C Believers should always pray and not give up, trusting that God will bring about justice. D Widows are more likely to have their prayers answered than other people.