Which of These Best Describes the Reason Islam Is Divided

Which of these best describes the reason Islam is divided into two distinct denominations?

A) After the death of Muhammad his followers disagreed on how to choose his successor.
B) There were disagreements about which practices should be considered orthodox.
C) After the death of Muhammad there were two locations with high populations of Muslims.
D) Something similar to the Protestant Reformation occurred in the sixteenth century.