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

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

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