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

A God only answers prayers that are spoken loudly and publicly. B God is like the unjust judge and needs to be worn down by constant appeals. C Consistent and unwavering prayer is necessary, even when answers seem delayed. D Prayer is only effective if it is offered in a specific place, like the temple.