What Was the Primary Reason Why Russia Expanded South into the Caucasus

What was the primary reason why Russia expanded south into the Caucasus and east into Manchuria?

A) Russia was proving its religious toleration by integrating Muslims into its empire.
B) Russia wanted to acquire important deposits of rare minerals necessary for industrial growth.
C) Russia was responding to perceived threats from rivals in those areas.
D) Russia wanted to acquire new land on which to settle former serfs.