Because kiev was a major center of slavic population, kiev kept the slavs

under kiev’s powerful rule.
from being dominated by other groups.
from wanting to live in any place other than kiev.
under the influence of the eastern orthodox church.

Explanation:

Kievan Rus (862-1242 CE) was a medieval political federation situated in current Belarus, Ukraine, and part of Russia (the last named for the Rus, a Scandinavian people).

The name Kievan Rus is a present day (nineteenth century CE) assignment however has indistinguishable importance from 'place that is known for the Rus', which is the manner by which the district was alluded to in the Middle Ages. The Rus ruled from the city of Kiev thus 'Kievan Rus' essentially signified "the grounds of the Rus of Kiev".

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering