in c++

What is true about Polymorphism? Select one or more: a. You must add the override keyword somewhere to the right of a method declaration to indicate that you are overriding a non-abstract method b. You cannot make an instance of an abstract class c. Abstract classes are required for polymorphism in C++ d. You must implement all unimplemented abstract methods from a ancestor classes in order to make a concrete class e. You cannot make a variable using the type defined by an abstract class f. Specifying that a method is abstract happens as part of the class definition