Considering the cultural context of ancient Israel, what was the primary significance of 'gleaning' (leaving leftover crops in the field) as exemplified in the story of Ruth?

A It was a common practice to ensure efficient harvesting and minimize waste. B It was solely a religious ritual with no practical implications for the poor. C It was a legal obligation imposed by the Roman Empire to control food distribution. D It was a divinely ordained social safety net, providing sustenance and dignity to the poor and marginalized.