Read this: Rurik led the Rus' until his death in about 879, bequeathing his kingdom to his kinsman, Prince Oleg, as regent for his young son, Igor. In 880-82, Oleg led a military force south along the Dnieper river, capturing Smolensk and Lyubech before reaching Kiev, where he deposed and killed Askold and Dir, proclaimed himself prince, and declared Kiev the "mother of Rus' cities." Oleg set about consolidating his power over the surrounding region and the riverways north to Novgorod, imposing tribute on the East Slav tribes. In 883, he conquered the Drevlians, imposing a fur tribute on them. By 885 he had subjugated the Poliane, Severiane, Vyatichi, and Radimichs, forbidding them to pay further tribute to the Khazars. Oleg continued to develop and expand a network of Rus' forts in Slav lands, begun by Rurik in the north.

Who wrote that Kiev was "The mother of Rus cities"?
What is the answer? (If it cannot be answered, return "unanswerable")
unanswerable