Which of the following best explains why the red army won the Russian civil war?

A. although many groups opposed the Bolsheviks, the white army was not very unified and this made it harder to have success in the war
B. the red army used its red terror program throughout Russia in order to take out political opponents and strengthen themselves
C. the white army withdrew from Crimea and this allowed the red army to successfully fight them one last time
D. the white army ultimately did not have enough help from outside countries to fight the red army successfully

A. although many groups opposed the Bolsheviks, the white army was not very unified and this made it harder to have success in the war

This answer best explains why the red army was able to win the Russian civil war. The lack of unity among the various factions that made up the white army hindered their ability to effectively coordinate and fight against the Bolsheviks.