What Weakness in the Abbasid Empire was revealed when Tahir of Khurasan

This question is incomplete, here´s the complete question.  

Weakness in the Abbasid Empire was revealed when Tahir of Khurasan did which of the following?

A)  Invaded Baghdad and killed the caliph

B)  Declared the Umayyads to be the true caliphs

C)  Became governor of Khurasan and ruled it independently of Baghdad

D)  Founded the city of Cairo as a new trade center

Became governor of Khurasan and ruled it independently of Baghdad

Explanation:

When Tahir was appointed as the governor of Khurasan, a period of antagonistic relationships between the Tahirids and the Abbasids begun.

Tahir was determined to turn independent. Him dropping the al-Mamun’s name from the Khutbah and creating coins after his own name were all symbolic expressions of independence from Baghdad.

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