What is the long-term effect of the events described below?
a. the sunni and shi'ah settled on a common doctrine and are merged into a single sect today.
b. the difference between the sunni and the shi'ah have remained unresolved and still cause problems today.
c. the sunni and the shi'ah eventually weakened each other in a series of wars, and they are not significant forces today.
d. the differences between the sunni and the shi'ah were finally resolved, and the two sects present a united front today.