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

A The importance of legal rights and social justice for the vulnerable. B The need to avoid bothering those in positions of power. C The necessity of constant prayer and unwavering faith, even when answers are delayed. D The corrupt nature of earthly judges and the futility of seeking justice in this world.