PHP Basics Learning Mode

Which values should be assigned to the variables $a, $b and $c in order for the following script to display the string Hello, World!?

$string = "Hello, World!";
$a      = '';
$b      = '';
$c      = '';
if ($a) {
    if ($b && !$c) {
        echo "Goodbye Cruel World!";
    } else if (!$b && !$c) {
        echo "Nothing here";
    }
} else {
    if (!$b) {
        if (!$a && (!$b && $c)) {
            echo "Hello, World!";
        } else {
            echo "Goodbye World!";
        }
    } else {
        echo "Not quite.";
    }
}
?>