Which of the following statements are true?
Assume that class WildAnimal { } and its subclass class Tiger extends WildAnimal { } exist. 
        a) Using List<? extends WildAnimal> is a case of contravariance. 
        b) Assume the following class List and its use (E is a parameter type): 
             class List<E> { ... // list of elements of type E
               void add (E elem) {
                   // add elem to list
                   ...
               }
             } 
             List<? super Tiger> animalList = new List<>(); 
           
           It is possible to add an object of type WildAnimal to the list by calling:
             animalList.add(new WildAnimal());
            
        c) Using List<? super Tiger> is a case of covariance. 
        d) List<?> is compatible to a concrete List of any type.