How Did the Caliph Al-Mahdi Resolve the Problem of Succession

How did the Caliph al-Mahdi resolve the problem of succession in the Abbasid dynasty?

A) He specified the rule of primogeniture, the succession of the oldest son.
B) He accepted the Shi?a doctrine that only the person with the most direct relationship to Muhammad should succeed.
C) He accepted the radical proposition that only a man of demonstrable Islamic purity should succeed.
D) He failed to resolve the problem of dynastic succession with disastrous results.