Late Static Binding Question

latestatic.php

<?php

class A {
	protected static function doStuff() {
		return  "Hello ";
	}
}

class B extends A {
	
	public function helloWorld()
	{
		// Add one (1) line of code here that concatenates strings in order to 
		// output "Hello World!" using only the doStuff() functions

	}
	
	protected static function doStuff() {
		return  "world";
	}
}

class C extends B {
	protected static function doStuff() {
		return  "!";
	}
}

$object = new C();

echo $object->helloWorld();

// Write helloWorld