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) In class List<E>, where E is a paramter type, the method add is implemented that allows to add an element of type E to the list:
           
           void add (E elem) {…}
           Can you build a concrete List (list with concretized types) for adding and storing WildAnimal objects? 
        c) Using List<? super Tiger> is a case of contravariance. 
        d) List<?> is compatible to a concrete List of any type.