Why did a civil war break out in rome

The various civil wars of the Roman era were all caused by a power grab by some general or some politician with a strong military backing.

The most famous of these are the war in the 40s BC between Julius Caesar and the optimate faction of the senatorial elite initially led by Pompey and the subsequent war between Caesar's successors and die hard loyalists, Octavian and Mark Antony in the 30s BC. Following is a list of civil wars in Ancient Rome.

Ithink the answer is b