Why Did the Russian Expansion Policy Focus Particularly on Central

Why did the Russian expansion policy focus particularly on central Asia?

A) The Russians wished to seize control of the trade routes with China.
B) Most of the Russian population remained ethnically Mongol with clear cultural ties to central Asia.
C) There were natural barriers to westward expansion.
D) The Russians were motivated by a desire to push the former Mongol overlords farther back to prevent renewed invasion.