Overriding method with a lower-visibility one results in compilation error

I have one class and one interface:

public interface A {
    public void getNum();
}

public class B {
    public void getNum() {
        System.out.println("4");
    }
}

public class C extends B implements A {
    protected void getNum() {
        System.out.println("3");
    }
}

Now my question is, why this code is giving compilation error and how can we avoid it. Is there any way in which we can override this method in class C?