Inheritance and Method Call Errors

Given the following class definitions:

public class SuperClass {
    // default constructor not shown ...
    public void methodX() {
        /* implementation of methodX */
    }
    public void methodY() {
        /* implementation of methodY */
    }
}

public class SubClass extends SuperClass {
    // default constructor not shown ...
    public void methodX() {
        /* different implementation from methodX in SuperClass */
    }
    public void methodZ() {
        /* implementation of methodZ */
    }
}

Assume that the following declarations are made in a client class:

SuperClass s1 = new SuperClass();
SuperClass s2 = new SubClass();
SubClass s3 = new SubClass();

Which of the following method calls will cause a compile-time error?

I. s1.methodZ();

II. s2.methodZ();

III. s3.methodY();