The reserved word virtual is used with a base$-$class member function to alert the $C++$ compiler that it must determine the actual member function to call at run$-$time. Thus derived classes can have their own $($possibly completely different$)$ versions of the base$-$class function. When a base$-$class defines a member function to be virtual, what should each derived class do$?$

Derived classes should only be used as array elements

Each derived class should over$-$ride the virtual base$-$class function so that it defines a function with the same name that is unique to the derived class

Derived classes should invoke the base$-$class virtual function

Derived classes need to do nothing further