What enabled the Mongols to invade Kiev?

Russian rulers were not strong enough to reunify Kiev.

The element of surprise worked in the Mongols' favor.

Kiev's territory was far too enormous to defend.

The Russian military gave up and surrendered.

The Russian military gave up and surrendered.

This is the best answer from all of the choices because in history, the battle was heralded in the Kalka River back in 1223. It resulted in the Mongol rule over the principalities of the Russians, they actually retreated at first but an invasion plan followed. They only were victorious when Khan died.