6. Which of the following statements is completely true?

(a) If a class is declared to be abstract, then every method in the class is abstract and must be overridden.

(b) If a class is declared to be abstract, then some methods in the class may have their bodies omitted.

(c) If a class is declared to be abstract then all methods in the class must have their bodies omitted.

(d) If a class is declared to be abstract then all the instance variables must be overridden when a concrete class is derived from the abstract base class .