What are four valid examples of polymorphic method calls?

Given:
10. abstract class A {
11. abstract void a1();
12. void a2() { }
13. }
14. class B extends A {
15. void a1() { }
16. void a2() { }
17. }
18. class C extends B { void c1() { } }
and:
A x = new B(); C y = new C(); A z = new C();
What are four valid examples of polymorphic method calls? (Choose four.)

A.
x.a2();

B.
x.a1();

C.
z.a1();

D.
z.a2();

E.
z.c1();

F.
y.c1();