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] =&gt; red [green] =&gt; green [blue] =&gt; blue [yellow] =&gt; yellow )
• B. Array ( [a] =&gt; a [b] =&gt; b [c] =&gt; c [d] =&gt; d )
• C. Array ( [red] =&gt; a [green] =&gt; b [blue] =&gt; c [yellow] =&gt; d )
• D. Array ( [a] =&gt; red [b] =&gt; green [c] =&gt; blue [d] =&gt; yellow )