In the Parable of the Persistent Friend (Luke 11:5-8), what key aspect of prayer is Jesus emphasizing through the friend's eventual willingness to help?

A God only answers prayers that are offered at midnight. B God is more likely to answer prayers from those who are wealthy and powerful. C God responds to persistent and bold requests, not because of obligation, but because of the petitioner's earnestness. D God only answers prayers that are perfectly worded and free from doubt.