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.

explanation: we can only guess at how our actions may be perceived in the future. doing something really big like killing lots of people, saving lots of people, affecting the lives of lots of people is a sure fire way of being remembered in historical records. we can also make a difference in the history records by being a small influence on something bigger, whether that is as a civil servant, scientist or artist.

of course you could argue that we all affect history just as tiny cogs in a   bigger machine.

if you mean how do people influence our opinions of things which have already happened, then simply by writing a book which looks at things form a different perspective and trying to convince people that this perspective is more accurate or truthful. people do it all the time i.e. richard the third was a terrible or was he? plenty would disagree, it's just that their views aren't the most widely known.n:

during the 20th century, the role of the vice president has evolved into more of an executive branch position. now, the vice president is usually seen as an integral part of a president's administration and presides over the senate only on ceremonial occasions or when a tie-breaking vote may be needed.