What will be the output of the following PHP code ? <?php $a1 = array("a" => "red", "b" => "green", "c" => "blue", "d" => "yellow"); $result = array_flip($a1); print_r($result); ?>

• A. Array ( [red] => red [green] => green [blue] => blue [yellow] => yellow )
• B. Array ( [a] => a [b] => b [c] => c [d] => d )
• C. Array ( [red] => a [green] => b [blue] => c [yellow] => d )
• D. Array ( [a] => red [b] => green [c] => blue [d] => yellow )