This question may be usual for many, i tried for an hour to understand the things but getting no proper explanation.
MSDN says, System.Object is the ultimate
Multiple inheritance means inheriting from multiple classes into one class. Your example is valid, but you wouldn't be able to do the following:
Class B : A,C { --- }
A definition for multiple inheritance:
"Multiple inheritance is a feature of some object-oriented computer programming languages in which a class can inherit behaviors and features from more than one superclass."