Which are correct uses of generics (here, R,S,T are type parameters)?

        a) 
        class A<T,S> {
          T x;
          R y; 
        }
        b) 
        class A<T,S> {
           S x;
           T y; 
        }
        
        c) 
        class A { }
        class B <T extends Throwable> extends A { }

        d) 
        class A extends Thread {}
        class B<T> extends A { }