In the Old Testament, what was the primary purpose of the 'cities of refuge,' and what does this reveal about ancient Israelite concepts of justice and mercy?

A To serve as military outposts against invading armies. B To provide temporary sanctuary for those who had accidentally caused a death, offering protection from blood vengeance and reflecting a balance between justice and mercy. C To function as centers of trade and commerce. D To act as prisons for criminals awaiting trial.